An M16 mag?
First of all; give it a dose of percussive maintenance against something solid (yes, seriously). Sometimes things can just get stuck and need a good whack to free things up.
If that doesn't help, look at the bottom of the mag & undo the screw there. You should then be able to just pull the inside of the magazine out through the top; you *may* need to push the two little tabs each side of it near the top in to do so, but it should just slide straight out.
Then it's just a case of manually spinning the gears inside to see what the problem is; usually when a hi-cap stops feeding for me, you'll be turning the main wheel and see the small brass gear won't be spinning; or will only be spinning occasionally.
Fixing it may need you to take the insides apart completely; but we'll come to that later.