i had one about 9am yesterday, came back on about 10pmish and there was still one.. dunno if its the same one, or i was just unlucky, but coudlnt get on at all yesterday...
Well, To my knowlage, One of them is when the PHP extention crashes, And One is when the mySQL DB Crashes. I think you can go on the MAIN screen when PHP crashes, because Apache has Cached it.