Dell Studio XPS 7100 Won't Turn On

I'm stymied.  I opened my Studio XPS 7100 to clean it.  (It is out of warranty.)  I was specifically interested with cleaning the optical drive, which is not functioning properly.  Anyway, got it open, removed front bezel, cleaned it up inside.  Put the cover back on and hooked everything back up to turn it on so I could retract the optical drive tray to put the front bezel back on.  It booted up perfectly fine.  I even waited until it finished booting and then powered it down properly so that I could disconnect stuff to get the front bezel back on.  Powered it down, pulled off the cover, reattached the front bezel and put the cover back on, plugged everything back in and now it won't power on!  I took the cover back off and stared inside for a while, but since I didn't touch anything inside, except carefully blowing off dust with some compressed air, I have no clue what it could be.  Any ideas?  I can't seem to get to the power button assembly to see if I might have jarred something loose, but all the other connections inside look fine.