Looks like the recoil spring and guide rod have slipped through hole in slide.
First- get slide off and recoil spring seperated.
Second-hold slide upside down with barrel in place so chamber is closed.
Third- put recoil spring in place ..rounded end in hole in slide, flat end on step of lug on barrel
Fourth- position slide at front of frame and slide back over lugs on frame.
Let us know how it goes, if you have trouble I will see if I can provide step by step photos.
Good luck, be safe!