Q: Yet again Following a Yosemite Upgrade my imac will not start - I rely on this for my teaching - Yes should have learned my lessons previously Any help apreciated

I have an imac late 2009 on Yosemite core 2 with 8 gig ram

The Yos upgrades seem to have a real issue with my mac. I have an off white screen and left it that way for hours but nothing has happened. Should I just leave it alone as was suggested following the previous initial install of Yos?

Thanks in advance - Aindre