Can I be sued over a written contract (no lawyer) from 20 years ago?

Anybody can be sued for anything, but whether you can have the suit dismissed on statute of limitations grounds depends on when the alleged breach occurred.  For example, you can have a 99 year lease entered into 94 years ago.  If you fail to pay rent in the 95th year, you can be sued.  Your friend could have a claim against you (and both of you would have claims against the friend who never made his payments) for your share of anything he had to pay as one of the co-signers.  HOWEVER, when your friend filed for bankruptcy (I'm assuming it was chapter 7), his assets, including any claim against you, became part of the bankrupt estate and owned by teh bankruptcy trustee.  He no longer owned it, just like he would no longer own a vacation home or other non-exampt property which would be sold and used to satisfy his creditors.  It's as if he had a promissory note which he never let the bankruptcy court know about, and now that he's out of bankruptcy, without having  paid his creditors with the promissory note, he wants to try to cash it.  He can't because it doesn't belong to him anymore, it belongs to the bankruptcy trustee.