What do you do if you have a cavity under your bridge?

Usually bridges need to be replaced at least every 10 years, there are some exceptions, but imagine something working 3 times a day 365 ,with stress, so there is always a chance of needing an adjustment, also receding gums, can cause a cavity on the root of the tooth, seeing there is a cavity it's time to change it, you should think about getting a dental vacation, relaxing and changing that bridge,for a new one.

If you get a cavity under an existing bridge you will need to have the entire bridge replaced with a new one or have a dental implant placed.  If you choose a dental implant for the missing tooth you will need two new crowns for the teeth that supported your old bridge.   Good luck.