Why does my computer keep restarting itself ? HELP !?

Then when it loads back up a message will appear saying 'System Has Recovered From A Serious Error' and then ask if you want to report the error . Why does it keep doing this ? It doesnt matter if in on the internet or if it just loads up it keeps doing it ! and its not overheated its still cool ! and the fan is working , its plugged in properly . HELP PLEASE !