The first thing to try is to shutdown the PC and then restart it - this solves many problems.

Boinc back running after shutting down and restart.
Thanks you Cruncher!!

Does it say in the Event Log that it stopped because the CPU is busy, or another reason?

'have not had a problem (like this)' replies does not get you anywhere, so let me direct. Go to the boinc datadirectory, usually c:\programdata\boinc\ but the exact path printed at the beginning of the event log after a restart, and look for the stdoutdae.txt file and scan for when the computing stopping messages appear. Copy from there back up to the previous time computing stopped. Post the whole section in a reply so we can read what your agent is doing. If we don't get a response, the problem is considered DIY resolved as so often happens. This includes the one of 'fixed it' without telling what was fixed and nobody the wiser, no future help for users with the same issue.