Hi Chris,
There might be various things:
1 - The database connection was lost for some reason;
2 - The site has used all memory available to it;
and more I think.
If you have a server I would look into the Windows Logs (Administrative Tools/Event Viewer) to see if there is anything reported there. If you don't have your own server you can still bring this up with your hosting company and they should be able to check the Windows Log for more details. Those details would have to pinpoint the problem.
Cheers,
Aderson