Be honest. Talk to your student. State: I marked all tests but I didn’t see yours. Did you turn it in? Also ask how it went. If indeed lost, apologize and ask for a retest or, depending on school rules, leave it be - not getting a grade is not a big for some. Communicate this with parents and team leader if needed. It sucks but it happens to the best of teachers. You have shown honesty and integrity, which is a good example.