Today I got a 'crescent' 3/4 size with a loose bridge.
How would I fix it?
I recommend finding a repair person to fix this. If you don't have the proper tools, you might break it more.
But, if you truly want to try, This video might help. He basically takes off the strings, bridge, and string pegs. Then, he glues the bridge back on, after taking the old glue and crud off. This guy IS a professional who has obviously done this before. I recommend against trying, go find a repair person.