Legal validity of a verbal agreement?

I paid over $3500.00 in dental work to have my girlfriend’s teeth fixed after she was assaulted. She insisted that she repay me. When I refused to let her pay, she signed an I.O.U. for that amount, and I took it.

A year later, we split up, and I let her take the dentures and molds that I paid for on the promise she would repay.

I have since lost the I.O.U., and now she refuses to pay up. She doesn’t know I have lost the I.O.U., and I have threatened legal action if she does not repay in full within 30 days.

What are my options here? Any help would be appreciated. Thanks!