What could happen if screws become loose in ankle?

That would: Depend on how long they were in and if they completed their function yet.

Pain infection: If your screws become loose in your ankle then it may cause you pain or some other symptom like numbness or opening of wound. Usually screws only need to be in for a certain amount of time in order for your bone or other structure heal. Usually the screws are not necessary after healing has taken place. If it's a concern hardware can be removed. Loose hardware in a joint can be a serious concern.