I found out that some important office documents have gone missing from my custody. I am responsible for the lost documents. What should I do in this situation?
If you think there is a chance you can find the documents then search for them now.
If you are pretty sure you wont be able to find them then report it to your boss ASAP.
Don't try to hide it. Tell them it's your fault and show them you are willing to do whatever has to be done to rectify the problem.