Is multiple possibilities that you pretty much have to eliminate one by one. As you surmise, is possible something failed in the scope or the scope mounting while it sat in storage - hard to say?? Also, the bedding in your rifle could have changed - dried out or took on moisture - maybe check the action screw torque - too tight or too loose? Clearance for the barrel - I do not know how CZ rifle barrels were originally bedded? Or, even, you, the shooter, may have changed a bit. I have a number of extra scopes here - so swap out to another scope - is what I would do first, because I can. See if that makes a difference. If you know another decent shooter, get them to try it - for sure with my son and my deer rifle off of sandbags at the range - same rifle, same scope, same box of hand loads - he would struggle to get 5 rounds at 2" or less - I would do 1" to 1.25" pretty consistently - so the shooter is part of it as well.
In desperation, I have removed the scope and used just the iron sights - at 25 yards. Most shooters should be able to get the holes very close to touching each other at that range. In this case, I was getting 4" 3 shot groups with the irons - at 25 yards - the "fault", so far, is thought to be the .005" oversize groove diameter that was found by slugging the bore - we never did have that rifle shooting well - no clue how the previous owner hunted with it - or at least hit anything with it? So, starting to get fairly esoteric - check that there is no corrosion within the bore, or damage to the crown.