Rear shock playing about!

Hello
Just wanted to run this past you.I took the car for a quick trip today and had a feeling that the rear left shock felt loose.
Got the car home and the top of the shock(threaded bit) had pushed up through the rubber cover!.
When I pushed down on the rear of the cat the shock would push up further?
The only thing I can think of is the snap ring which is normally on the piston has done one.Im sure I have reassembled it correctly,Has anyone ever had the same problem or is it time for new shocks!.
Thanks Sean

get em off and have a look, often its the circlip thats gone missing but it could be the upper spring plate has rusted thru. there should be a metal ring pushed up to the collar which then pushes on the metal cup