Missing chameleon claws are usually caused by the chameleon climbing on the screen walls. The screen holes might be too small, and the chameleon's claw could have gotten stuck. Fungus and Bacteria can also cause nail loss.
What does the foot look like? Is it swollen? Is she using her foot as normal? The nail might grow back, it depends on if the base is still there. You should watch the area closely. Try and keep it clean; many chameleon owners recommend applying a small amount of polysporin daily on the area of the recently lost claw. Your local vet might have a recommendation. If the foot becomes very swollen or discolored, you will need to have him treated for infection.
One missing claw will not greatly affect the chameleon, but multiple nail loses can hurt her ability to grasp onto twigs etc. What kind of cage are you using?