My blade on my riding lawn mower wont spin?? PLEASE HELP!!?

I have a MTD 12.5 hp riding lawnmower, fairly new. I bought new blades for it but the previous ones were so tightly ceased on that I couldnt get i off. So I decided to just sharpen my old blades that were on it. The blade wouldnt stop moving while I was sharpening it so I stuck a screw driver in this whole in the deck to stop it from spinning. Once I started sharpening it I heard a ping and a spring popped out. I cant figure out where this spring went. Now when I put the deck to go down and start the blades the blades dont work. I think it has to do with the belt tension because the belt is very loose and that is probobly why the blade isnt moving. The only problem is that i dont no where the spring went. Any suggestions????