Contact your host, they need to restart the SQL service or maybe even do a little more. If they try to say it's your fault, check in cPanel(or your control panel) and see if you still have the right SQL database for the forums. If you do, ask them nicely to restart the servers SQL service from WHM.
WHM stands for Web Host Manager. It's your hosts admin control panel to the server, basically. Inside WHM there's options to restart server services such as SQL, etc. They just click it and it restarts.
Just incase your host doesn't know what you're talking about when you ask them to restart SQL, here's what the option looks like in WHM. They can access WHM from http://theirdomain.ext/whm/. They should really know all this stuff though.