More information would be very helpful. Did it work and suddenly stopped working? What email client are you using? Have you changed any settings anywhere? Do you have your email account set to keep your emails on the server? In some settings once an email has been viewed it's not available on other devices. Provide as much information as possible please to give a better assessment of the situation.
Phone type, email client, how it's set up, are you seeing it normally on the pc?what email app on the phone? Android? Etc

If an Android phone, make sure that synchronization for that particular app is on. Look at Settings, Sync (or some similar language) to be sure that it's enabled for your mail app.

If you are using Yahoo Mail, mine has been acting erratically the last few days. Multiple times during the day, it's not current. When I manually trigger a sync, it doesn't sync. I also uninstalled-reinstalled and nothing improved.

I've experienced this before with the Yahoo mail app, usually every few months, and it passes. Their app is pretty crappy compared to competitors. I'd use my Gmail account more often, but I don't like the conversational layout of email threads. There are third party email apps you may want to try from the iTunes or Google Play stores, I haven't found one I like.

Wait a few days, see if it gets better. In the meantime, you can bookmark the web page access to your email in your phone's browser app, that should work fine. Except you'll get no notice of incoming messages.

It's AOL and Hotmail. Haven't changed anything. It's been going on for a couple of months now. Even if I have unread mail on my computer it doesn't show up on the phone. Tech support said all they can do is to reset to factory setting which would mean losing everything. No way ! I will try to sync but I think it already is synced. Sigh...

There's nothing saying sync. When I try to set up AOL it says authentification failed.

Authentication failed sometimes means a problem with username and password. Try taking the phone in to a retail location for your carrier, see if they can help.

A reset isn't that big a deal. The apps can get reinstated quickly. If you have important files on the phone, you should have them backed up elsewhere anyway.

I explained where the sync entries are made, is, you read through too quickly.

There's nothing anywhere about sync. It's a Galaxy Samsung. Android. General settings, nothing about sync. Mail settings, nothing about sync. Have not changed password.

I changed my password on my computer and downloaded the new AOL app on my phone. All is well. Yay ! Thank you both.