I'm pretty sure it's my computer. Any ideas on what to do? I'm running Zone Alarm, but it
is also running on my office computer & that works fine.
Try adding your cpanel domain into your firewall permissions. Also, clear out your browser cache as you may be loading a broke page that never gets updated and delete offline browser files.
Also look in your firewall if you have any alerts relating to cpanel and its domain. If so, delete it and access cpanel again to allow it to ask for permission.
In my past experience from clients using zone alarm, I know merely disabling it for testing may not relase appropriate rules until the computer is rebooted.