You need to visit a dentist to evaluate the condition of the cap as well as the tooth on which the cap was fixed. The commonest cause of loosening of a cap is secondary Cavitation at the interface of the cap and the tooth. This can even make the tooth non-restorable and the cap cannot be fixed again. If the tooth is intact, the same cap can be fixed if the cap is also intact, otherwise a new cap can be fabricated. Regards.