The sear bar spring is what returns the trigger to the ready position. The spring may be broken, but it is probably a jam of the sear bar. Have you tried pulling the trigger forward to reset it? It is possible that while holding the takedown plunger in to install the barrel you jammed the trigger back, I have done it recently.
Remove the barrel, pull the trigger forward and then re-install the barrel. If that doesn't work remove the slide or side plate, (slide is easier) and manually trip the sear to release the hammer. The trigger should return to the ready position.
Let me know how you make out with it or if you have any other questions.