Several
First try carefully screwing the whole thing back in...be sure NOT to get it crossthreaded...and to use a screwdriver that fits the slot correctly. Once you have done this and tightened it, try tightening it a little past "tight enough" then a fast twist back along with downward pressure and loosen it. This has usually worked for me...kinda difficult to demonstrate the correct "body english" but try it first.
If that does not work, you can either try and find a gunsmith with a staking tool, or try and make one yourself and re-stake the bushing from inside the frame...sort of hard to do with the grip on. And last, the old timers would sometimes put a drop of iodine on the threads and wait for a day or two until some rust had set in...you could probably do the same with some loctite.
Once it is off (probably method #1 will work for you) if possible, re-stake it and use a little clear fingernail polish or some loctite to be certain it does not happen again...ditto for the others if they seem loose also.
Good Luck