stand at back of desk top

the stand on the back of my lap top does not hold tilt positions

Then a component inside the iMac has broken. Take the computer to your local Apple Store and they should replace it. If the computer is out of warranty ask that it be covered. You can tell them there have been hundreds of reports of this issue on the Apple forums. If the tech does not help, ask for the store manager.

Be polite, organized with all receipts and persistent.