repairing the split follower action?

I have recently purchased a second hand door and i just tried to change the spilt follower action so that the handle on the inside rather than the outside retracts the latch, it worked for a day but now it appears that the pin has disappeared. is it possible to repair, if so then do i need to get someone out to look at it or can i do it myself?

Thanks for any assistance you can offer.