My mother lost her life insurance policy, and didn't tell us until now. Can I still prepay in payments?

By "lost" do you mean she can't find the paper document? Or that it lapsed for lack of payment?

Do you know who her agent is? If so, contact that person. If not, contact customer service for the insurance company. Explain the situation and ask for another copy of the policy (if the document is lost) or what you can do to reinstate the policy if that is the problem. You may need your mother to get on the phone and give permission for them to talk to you, or they may ask for the request in writing, but I'd start with a phone call.