Welcome to the forums.
You may want to make sure that all the Zimbra services are running. Run the following command on the server, as the 'zimbra' user.
If there are any services that are not running, you can restart the server by running 'zmcontrol stop' then 'zmcontrol start'
Are you accessing the same FQDN for both the admin panel and the webmail?
You may also want to check your
Log Files - Zimbra :: Wiki to see if there any errors.
If you can provide some more info, it'll be easier to figure out what's going on.