Using the Information button above or Colt AR15 manual for reference, ya may have to remove the collapsible stock in order to get things back to normal here, but this would be my last resort.
First the poster above could be correct, as blown primers can and do cause such lock-ups. Normally if I thought this had happen, I'd probably try the "bashing" the stock on the ground technique to see if I could loosen the hung primer, allowing the bolt to return forward.....
There is a possibility the buffer retaining pin has come unhinged at the rear of the lower and could be hanging things up.
If whanging and bashing the stock on the ground make no change to your present condition I'd then remove the stock and buffer tube and attempt to clear this jam from the rear.
Good luck and don't forget the assembly-dissassembly threads noted above if ya need 'em.
Mike