Did it break off (more likely) or come unscrewed (much less likely)? Look down into the top of the lift arm, do you see a bit of a nub centered in it? A new intact lift button looks like this (photo stolen from CCS):
I broke the top off of one on my parent's 125 back when I was a teenager. Luckily for me the narrow portion stayed with the lift rod and I was able to unscrew it with a needle nose pliers. Granted, the tractor was not that old back then and rust had not set in. If you just need to use it, any thing you can press down on it with will work, I used a screw driver back in the day on the one I broke until we got the new part.
