If you are on a shared server with other web sites sharing the same server you all use the same resources, if someone on that server uses up all the resources it screws it for everyone.
If you are on a dedeicated server with no other sites at all on the same server (this is very expensive) and you get this error, you need to check that all applications running on your site are releasing server objects aned closing database connections.
The forum software does release all objects and close all database connections so the forum sofware should not course this trype of problem.
Try rebooting the server if you can.