Not only is my selector stuck in the firing position but I can not pull the trigger almost as if it was stuck in the safety position.  It is a Bushmaster lower and this is the first time it has happened.  I have only replaced two things on my rifle, I added an LMT enhanced bolt(I did check the headspacing using a go gauge) and I put one of the shorty butstocks on it which replaced the pinned CAR stock.

My guess is that you blew a primer, and now a piece of either the cup or anvil is stuck under the trigger.

If you can't get it out by using a pick, then you will need to pull the FCG and clear it that way.

www.ar15.com/content/guides/assembly/lower/