Simple fix!!!!!Re-install the old scope?Try your gun.If groups return,then you know what it is,right?
Also,have you given the barrel a good cleaning???

10-4 on that. that was my first responce. if all you did was change the scope,then most likely thats the problem.
start there first.

My guess is that there is nothing wrong with your rifle. First clean it. I use Wipeout sit for 2 hours, patch til dry and clean.....the first patch should be dark blue which indicates copper build-up. Followed with Butch's Bore shine 20 scrubs with brush....patch til spotless. Re apply Wipeout for 1 hr and patches should be clean. Now put the old scope back on tighten everything and shoot it. If you are GTG then put the new scope back on and check the groups....If they are GTG you're a happy camper....if not, document your groups and send the sucker back for a new scope. If you have a wood stock, make sure nothing is binding anywhere. Some stocks can shift/warp with age. Good Luck, Rick.

The scope is obviously the first thing to check as others have already suggested.A good bore scrubbing is also in order.Are you using the same boxes of ammunition that you used last year or are they different boxes of ammo using the same bullet?Occaisionally the ammunition companies change loads slightly which can have a big effect in some guns.

Well I didn't find anything loose on the gun, even after attempting to clean(no signs of copper build up or even regular fouling from firing. I think it might have been the boxes of shells I had. The one and a half boxes were two years old and after I shot all of them and borrowed three of the same exact thing only from a freah box a friend had, I had a one inch group again. I don't know what happened to the bullets to make them shoot this poorly, but my new box shoots just fine. Thanks for all the advice and help.