Do I need a signed contract in order to sue for money owed to me?

You don't need a signed agreement to sue someone to repay a loan you made them, although it would obviously help you prove your case.  It is not clear to me whether you meant that you had filed bankruptcy, or the debtor had, but either way you can't now sue for a loan defaulted on prior to the bankruptcy (assuming that you are still within the statute of limitations anyway).  If you're referring to your friend's bankruptcy, the debt was presumably discharged in the bankruptcy.  If you're referring to your own bankruptcy, which I am assuming was a Chapter 7, any receivable you had became the property of your bankruptcy estate, to be used to pay your creditors.  You were required to list that asset in your filing, with potentially severe consequences if you didn't.  It is only if the trustee of your bankruptcy estate knew about the claim and declined to pursue it (or if you paid back all your creditors in full) that you might have a right to do so yourself.