I'd check to see if, before cocking the hammer, the trigger was moving freely. If stuck or the return spring isn't returning it briskly, the rifle action is due for cleaning and lubing.
If the rear stock is loose in the metal, it's dried out a bit from storage in a warm place. I'd remove the large screw behind the hammer with a close-fitting screwdriver. The stock should then slide off.
Rule Number 1: keep the WD-40 away from the internals of any gun! It will gum everything up within a year or so!!!! (If you must use it at all, apply only on outside metal surfaces.)
Okay, if you know how to disassemble and clean the action without buggering screws and pins, go to it, but if not: Get some Gun Scrubber and spray the internal action, hammer pivot, and trigger pivot parts liberally and let it dry. Apply good gun oil or synthetic motor oil to pivot points of hammer, trigger, lifter, etc.
If there's rust on exterior surfaces, use Break-Free CLP on #300 or #400 steel wool and rub gently. Depending on the extent of the rust, it could take time.
If the bore is clean and un-rusted, run a patch through it with Break-Free CLP or bore cleaner and check the patch for rust. IF the bore isn't rough, but just has rust color on the patch, use a bronze bore brush with Break-Free CLP to remove the rust. If its badly pitted, don't ruin it, but take it to a gunsmith or knowledgeable gun nut to check it further. (Pitted, but otherwise rust-free barrels aren't necessarily unsafe or unshootable, just need more cleaning after shooting. Have it checked by a gunsmith before shooting it.)
If the wood finish is very dry, consider sanding and using Tung Oil, or other oil stock finish. Follow directions.
Hope this helps. Everbody has their own methods of cleaning and rust removal and you'll probably hear from them.
“God save us from people who mean well.”
― Vikram Seth