On Linux, check to make sure the mysql daemon didn't die. If you have shell access, from a command line:
ps -ef|grep mysql
Should return at least two instances of mysql. If not, the service needs to be restarted. Google start mysql linux for detailed instructions.
Or on Windows, restart the mysql service. Google for instructions if you have desktop access to your server, otherwise let your hosting service know.
I manage a corporate intranet with tons of php/mysql apps, and that message invariably means that mysql has crashed.