My product was displayed at the exhibition, but someone else applied for a patent.

This situation belongs to the disclosure of use, that is to say, for the patent applied for by others, you have already disclosed the use before he applied for it, which constitutes the prior art of the patent. Therefore, you can submit public comments to the State Intellectual Property Office and submit the proof you exhibited to the State Intellectual Property Office. In this way, his patent will have no novelty and cannot be authorized.