The 27inch iMacs have a common problem where the spring in the stand breaks, you'll hear a crack and you screen will just tilt down.
When it broke on my machine I considered getting it fixed but that ran the risk of losing my data (like any time you return the machine fix a fix) so I opted to design my own fix.
My solution is a little man who holds the screen up - He holds the iMac 65mm up but he can be scaled up slightly to 70mm if you want your screen pointing up slightly.