Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
If the printing was 2-sided and one sheet is missing, then you should probably look in the rear or 'duplex' part of the machine.
Or perhaps the paper is in the underside of the machine, also, that would require a special approach to find the paper.
Does your printer model have a removable rear duplexing unit? if so, remove it by unlatching it, and see if it contains the missing sheet of paper.
Or open the rear door of the printer, if it does not have a removable rear duplexing unit.
Does this sound like the right track? let me know, and I will advise you further.