I have no answer about whether you will lose your mail but I did find some troubleshooting at Microsoft. I don't understand exactly what you mean by re-boot either :-)

Here are the recommended steps:

SYMPTOMS
When you start or use Entourage for Mac, the program stops responding (hangs) and you see a spinning wheel.

RESOLUTION
To resolve this problem, follow these steps:
Step 1: Use the Shift key
Hold down the SHIFT key, and then double-click the Entourage icon to start the program. After Entourage starts, clear all messages from the Outbox.

Note Holding down the SHIFT key when you start Entourage prevents the program from trying to send or receive messages when it starts. If this did not resolve the issue, go to the next step.
Step 2: Reset the Entourage preferences
Restore the Entourage preferences file to its default settings. To do this, follow the steps for the version of Entourage that you use.

Note This action removes any customizations within Entourage that you have set, such as customized toolbars.
Remove top-level preference files
Remove the Entourage preference files from the Preference folder. To do this, follow these steps:

Quit all Microsoft Office applications.
On the Go menu, click Home.
Open Library.
Note The Library folder is hidden in Mac OS X Lion. To display this folder, hold down the OPTION key while you click the Go menu.

Open Preferences. Drag the following files to the desktop:
com.microsoft.entourage.database_daemon.plist
com.microsoft.entourage.database_utility.plist
com.microsoft.entourage.office_reminders.plist
com.microsoft.entourage.plist
com.microsoft.entourage.syncservices12.plist

If you cannot locate any or all of these files, Entourage is using the default preferences. Go to "Step 3: Rename the Microsoft User Data folder."
Test Entourage. If the problem is resolved, follow these steps:
Quit Entourage.
Select and drag one of the files you moved to the desktop back to the Preferences folder.
Open Entourage and test.
If the problem does not occur, repeat steps A through C to move another file back to the Preferences folder. If the problem does occur after moving a file back to the Preferences folder, drag the specific file that caused the error to the Trash.

If the problem continues to occur, go to the next step.
Step 3: Rename the Microsoft User Data folder
Rename the Microsoft User Data folder to Microsoft User Data_Old, and then test the program to verify that it is working. To do this, follow these steps:

Quit all programs.
On the Go menu, click Home.
Open Documents, and then click Microsoft User Data to highlight it.
Press ENTER to select the folder name, move the cursor to the end of the folder name, type Old, and then press ENTER to rename the folder.
Open Entourage.

If the program starts, the problem involves the Entourage (Microsoft User Data) database. To resolve this problem, follow these steps:

Still in Documents, drag the newly created Microsoft User Data folder to the Trash.
Rename the Microsoft User Data_Old file to Microsoft User Data.
Back up the Identity.
Rebuild the Identity.

To back up or rebuild the identity, follow steps 3 and 4 in this Microsoft knowledge base article:

268322 How to rebuild your identity database in Entourage 2008 for Mac and Entourage for Mac 2004 to resolve most problems

If Entourage did not launch, create a new user account, and then start Entourage by using the new account.

Sometimes, specific user information is corrupted. This problem can prevent a program from installing, starting, or running correctly. To determine whether this is the case, you can log on as a new user or create a new user account, and then test the program. For more information about how to create a new user account, click the following article number to view the article in the Microsoft Knowledge Base:

2439218 Testing your user account to troubleshoot Office for Mac applications

If Entourage starts when you use the new account, try to start the computer in safe mode to see whether any third-party fonts or programs are running that might be preventing Entourage from starting.

For information about how to restart the computer in safe mode, click the following article number to view the article in the Microsoft Knowledge Base:

2398596 Perform a clean startup (Safe boot) to determine whether background programs are interfering with Office for Mac

If Entourage does not start when you use the new account, remove the Microsoft Office suite from the computer, reinstall the suite, and then apply all available updates for your version of Office. For more information about how to remove Office, click the following article number to view the article in the Microsoft Knowledge Base:

2500821 Manually remove Office 2008 for Mac completely from your computer

Here is a link that might be useful: Entourage for Mac hangs and does not respond

Thank you for your help, pk. We have a mac, (which I forgot to mention,) and I did find the above on google. However, I don't have a "hanging" problem, it's just not importing any emails nor allowing me to send any. We have tried quitting Entourage and restarting, but no luck. I can't seem to find anything online to help me with this; the problem started when Yahoo Mail had problems earlier this week, which is the service we have to use. I prefer the layout and speed of Entourage, which is a pop account that comes off of Yahoo. Apple Mail is working again, but no Entourage. Darn. Thanks anyway.

If I were you, I would ignore the word 'hanging' and try the steps anyway. What do you have to lose?

Thanks, pk. I'm stymied about "Go Menu." What and where is that? Perhaps I have a different version....I tried different things throughout yesterday, but no luck yet. I'm not terribly computer savvy, but can do the basics. I get frustrated when I can't find an icon or heading mentioned!

I am not a Mac user, but found an image on Google.

Here is a link that might be useful: Where is Mac's Go menu?

Thanks much, but I don't have that--must be a different version of what we have, which of course I can't remember at the moment.

If you follow the link, there are click-able links with instructions. If you can't follow the first step, just go to the next one. If it were an iPad, I would suggest simply deleting Entourage and reinstalling from the Apps Catalog.

Sorry, but I am unfamiliar with Mac, except the mobile devices.