I'm puzzled how that's even possible. The trigger pin has a cap on the left end and sets under the disassembly lever and as such can't walk out. Even with the disassembly lever open, it's still blocked. That lever has to be removed before the trigger pin can come out. Are you talking about the roll pin that secures the locking block?
As far as a fix, if it's the roll pin, you have several choices, drive it back in place and see what it does, replace the roll pin, or call S&W and tell them what's going on.
I know the pics are of my PC Shield .40, but my wife's OR Shield Plus is identical.