Ratcheting Type Chain Tensioners
I'll assume that you're dealing with a 300E or something similar?
Once the tensioner is extended, it has to be removed, taken apart and reinstalled. The plunger will only go out toward the chain, not back. You take it out and pull the plunger out the end. Then slide the plunger back in from the "outside" end, slide it in and tighten the retaining ring. Wait until the chain is connected to put the spring back in it. The threaded ring should be a 17mm Allen(it's been a while, maybe a 19 mm?).
If you destroyed the chain guide, the front cover has to come off. The oil pan just needs to be loosened slightly about half way back to get the cover off. If it just got chipped don't worry about it.