How do I fix a Bostitch nail gun that is jammed?
Oiling your gun on regular intervals is important. In your case, if the pin is stuck, try the following. 

Put several drops of oil directly on the pin and slide channel. Remove all nails, reassemble the nose and see if the gun will dry fire against a scrap piece of wood. the gun should fire and leave a dent from the push rod in your wood if it is working.

If the gun still does not fire, then the pin may be damaged or bent. At this point the upper air chamber will have to be removed and the pin assembly removed/replaced.  Rebuilding air guns is not real difficult and parts for a Bostich are readily available. You problem may be a worn or damaged O-ring in the compressor section. Rebuild kits for the upper end have got to be replaced often, especially if they have not been oiled regularly of after long periods of storage.  Parts are cheap. 

Some folks at the tool dept of Lowes or HD could be helpful guiding you. If you have a good lumber yard that sells Bostich tools, they often stock the parts and will be helpful in helping you fix your tool.

In my area, Bostich often have promotion displays at the vendors where they will do maintenance and install rehab kits at no charge with the purchase of a case of nails.  Both of my framers, Bostich 88W's, have been rebuild at least 4 times for free during these promotions.