Welcome to iMore! Your best bet is to delete your Gmail account from your device (this will not actually delete your emails from the server, so don't worry) and then add it back. Select "Google" under account type and the incoming and outgoing server information will automatically fill in for you once you log back into your Gmail account.
If you want to join the forum and reply back within this thread with the type of device you're using we can provide more specific advice.