I have a tooth on my bridge that is short when reaching my gum line , what's the best way to fix this problem?

Bridge: Fixing this depends on the type of bridge and where the bridge is located. If you are concerned because this area is visible and unattractive, the bridge probably needs to be replaced. If you are concerned because it is a food trap, it may be able to be repaired. In any case, the best recommendation will depend on a clinical and visual exam by your dentist.

New Bridge: BEST way is to have a new bridge made (presumably a fixed and not removable bridge). Can also add bonding to fill in the space or gum grafting to fill in the space.

New bridge: Whenever there is a space or gap between the edge of a crown or bridge and exposed tooth structure, that's an open margin. An open margin will eventually cause a restoration to fail due to bacteria being able to get in and under causing decay. You should have it replaced with a new bridge.