Assuming that the bridge is just beginning to lift ( ie a small gap at the back of the bridge ) as opposed to the whole thing coming away, it should be possible for a luthier to force some glue in there and clamp it overnight, should come out as good as new.
If the bridge/top joint has lost it's integrity, it will be necessary to remove the old bridge, clean off the two surfaces, prepare them for glueing and then clamping the whole thing until is secure.
In some cases the old bridge has come off due the drying out and warping out of shap, in this case a new bridge needs to be made.