Verified answer

Not clear if your strap came off the button or if the button itself came out of the guitar, but here's both.

If the strap simply slid off the button there's a product called 'Strap-Lock'.

This is a locking button device that will 'lock' your strap on to the buttons at both ends, and a set costs around $10.

Now, if the button actually came loose from the guitar perhaps is just came loose. In that case just screw it back in.

However, it the screw hole is stripped out you'll want to fill the old hole with a wood filler then re-drill the hole and install the button again.

I would strongly suggest you go for the Strap-Lock system which ever the case may be.

It's the best strap system out there and, for ten bucks, beats the heck out of dropping your guitar on the floor.

db