Sometimes the hook on the T-Bar (also called T-hook) of a GI Joe will break. To fix this, first take the joe apart. The top hook of the T-Bar is hooked onto the O-ring. The tricky part is the piece that goes into the legs. To fix this, one must remove it from inside the leg of the Joe.
Much like the joe back, there is a screw on the “inner thigh” area of the joe. Take a small philips head screwdriver and unscrew it. Remove the T-bar. The biggest trick to replacing these is that a T-bar is not something you can just find at a hardware store. Some online stores sell replacements (brianstoys.com sometimes has them in stock). Ebay is also a good place to look. Your easiest bet is to just take one from an extra or “beater” figure you have laying around.
Once you have the new T-hook in place, just reassemble to figure.