I don't know about Mac, but a windows computer has a hide task bar box that if checked will hide the task bar until you run a cursor over it. On my W 7 computer I right click on the start button and choose properties. A menu will come with task bar options on it. If you can find something like that remove the check mark. If that doesn't help, try googling "hidden task bar on a mac". I did it for you and here is a list of possible help on the subject.

Here is a link that might be useful: Hidden task bar

Are you referring to the dock? The toolbar is the menu @ the top of the screen - the dock is the thing w/ icons.The dock preferences in the Apple menu should enable you to configure that. Did you try clicking the little apple @ the top left of your screen?

Yosemite is so new, I imagine it has a number of bugs that will be revealed as more folks start converting to it.

Have you checked the Apple.com support pages & looked to see if there any discussion of this on the boards there?
If not, you can post a message & hopefully get answers there.

You can also try clicking 'Help' in the toolbar & searching there....

Here is a link that might be useful: Apple.com community support - Yosemite

Thanks for the replies but hats off to Carolb who nailed it. Thank you so much. Yes, I meant the dock. I had clicked on the little apple on the top left this morning but somehow did not notice the system preferences. Don't know how I missed it the first time around but I did. Just went back and found the system preferences and the dock option and found the option to hide the dock checked. It is now unchecked and all is well. Thanks again!

Abbey

You're welcome! I've found that something always seems to default when new software is installed, so my 1st troubleshooting move is to check on the preferences.