Well, it may help to have pics. Since you say it did not completely slide together, how far back did the slide travel before locking? Can you drop the hammer? If you can still see some of the levers, it sometimes just takes a 'concerted' push forward on the slide, and then release the hammer to try sliding the slide back on.
If truly stuck, it may have run up against the hammer release or other levers that raise when the hammer is cocked. May end up needing the grips pulled and hammer pin removed, not a task for a newbie.
Ok maybe i shouldn't have started a conversation like that, anyhow i have 2 p38, one is a 39 walther the other a 44 spreewerk, ice not fired either ,however clean ever year lightly, i inherited them 8 years ago along with several rifles and shotguns, in south carolina but i cant figure how to add to profile