We need a little more info to provide assistance. Can you access data on the device, good way to validate this is can you access web pages. We also need to know what kind of e-mail account are we having issues with? Is it a POP or IMAP account such as Gmail for before mentioned Yahoo, or is it an Exchange account from your work.
There's a couple of things we can check. First, go into menu>settings>accounts and sync>click your google(gmail) account>check to see if there are any red circles next to any of the items. If not, go into the Android market on your phone and see if you are able to search or download any applications. If this works too, try going to the following link from your computer and completing the Google Captcha unlock and then send yourself a test email. https://accounts.google.com/DisplayUnlockCaptcha
If none of these methods works, you will most likely need to reset your phone to factory default if this is set as your default account. You can try to remove your account from your phone from the accounts and sync screen, but if it is the default account, it wont let you and you'll receive an error message. If that's the case, you will need to complete a factory reset in order to remove the account from your phone and then re-sync after initial activation. This will erase any personal information on your phone including call logs, text history, applications and personal settings. Your contacts should be backed up to Gmail, but you can create a copy on your SD card by opening your contacts, pressing menu>import/export>export to SD card.
Let me know if you have any questions or if you are able to get Gmail working again.