95 C280 can't turn key in ignition after sitting a year?
 

I have this C280 salvage car/parts car/rebuilder that was fine the last time I started it a year or so ago but now I can't get the key to turn in the ignition switch. And I put a good battery in it. What could be the cause?

Wrong key or seized lock cylinder. Battery won't matter on that car, it should turn with NO battery in it.

Unless you mean a new battery in the KEY, in which case same answer, the key battery is only for the remote locking/unlocking functions, not the starter. Have you trid jiggling the steering wheel around? It might maybe have a locked lock cylinder because the sterring is jammed on to the steering wheel lock?
Gilly

I'll try jiggling the steering wheel and the key is pure mechanical.

OK, so you have like the valet key or something, the car has a remote lock/unlock, but each car was delivered with one valet key, which has no remote function.
Gilly

I tried jiggling the steering wheel with no luck. I guess my next step is to remove the whole lock switch assembly.

I sprayed some WD40 into the keyhole, jiggled the key in and out, right and left and sprayed some more. After about 10 times it worked. And it started right up too after sitting a year or more.