Getting Proofs
Getting hold of proofs is usually a struggle. If the work was in advertising chase the client for proof, they look good as they appeared as an ad in your portfolio.
If it was editorial in a magazine go out and by some copies again they look good as they appeared.
If you produced your work on computer then you will still have the image, if your posting the original get it copied first.
Return of Artwork
You should get your artwork back unless you have sold it to the client, unless you produced it on computer or you sent a scan. If they have the original, chase the client, the original is your property. If they have lost it, invoice them for about 50% of the original fee, tell them first that you are going to do this it might galvanise them into finding it.