Providing you have physical access to the server you should be able to open a terminal- Do you have physical access and can you open a terminal?
Make sure and check 'hosts.allow' that it contains:
Next check 'hosts.deny' that it contains:
I would then check my firewall, run 'iptables -L' and see if your IP you are SSHing from is being blocked.
Sometimes SSH access can be restored by 'stopping' | 'starting' your firewall too.
You may just have to go with a reboot though???
Hope this helps in some way?