Push the firing pin forward manually to check it, it's either a broken return spring, bent pin or just dirty, complete frame R&R info can be found in the FAQs trigger info by Perklo, you need to remove all of the pins(except the hinge) to remove the firing pin unless the transfer bar is broken. Brownells has the pin and maybe the spring. Might as well do a trigger job and replace the hammer spring while you're in there, Wolff EP springs are available direct from them, Midway and I think Brownells, links in the FAQs.
Tim