At the range today I popped a clip in my AR15 Sport, closed the bolt, clicked off the safe, and pulled the trigger - it didn't fire. I ejected the round and tried again - same thing. I looked at the rounds and no firing pin mark on the primer. Opened up the gun and the buffer retainer was lying beside the hammer - the buffer assembly was no longer contained within the buffer tube. The buffer retainer spring was nowhere to be found. I tried to replace the spring with one from a ballpoint pen, but the retainer wouldn't stay in place. First problem I've had in 2 years. Any idea what the problem is, and how do I fix it?