Make sure the main spring tension screw is tight. Thats the one in front, low down on the grip frame. If that don't fix it,remove the grips, put it a can pan with a 50/50 mixture of automatic transmission fluid and kerosene (available at hardware stores) let it sit for 2 days or so. Then, get a can of carb or brake cleaner and using the tube that fits the spry nozzle spray it in the openings by hammer while cock and uncocking, Same thing by the trigger. With cylinder open spray some in the little slot in recoil shield. Wait a few minutes and then repeat.

If that doesn't do it, let us know. If your not mechanically inclined it might nee a good gunsmith. But, if it is old hardened sludge the cleaner should get it. A loose tension screw can cause top part of mainspring and internal part of hammer to bind each other. If neither of those probably a damaged part on trigger or hammer.

Here is an N frame schematic
25-2 (N Frame) Schematic | Numrich

There a vids on U tube about assy

Good book is
The S&W REVOLVER - A SHOP MANUAL - Jerry Kuhnhausen

But, if the screw is tight and cleaning doesn't work. watch some vids, get some hollow ground screw drivers and I will try to help. Pictures can help