Isolating the cause of an accuracy degradation can be frustrating, and time consuming.
The first thing I usually do is to ask someone else to shoot the firearm, and confirm that I am not the cause.
I assume the gun has been cleaned at some point in recent history; if not I might suggest doing that and shooting again.
If the gun shot well at one time but doesn’t now, isn’t excessively dirty, and I have confirmed the shooter is not the cause then I would look at rifle bedding issues next. If the barrel was free floated, check to confirm it still is. If it isn’t free floated, has anything caused the stock to shift? If it’s wood, moisture could do it.
Check the bedding screws and confirm they’re not the cause.
Next I would consider ammunition. If you've recently switched ammunition, try a different brand.
Assuming it’s the same ammunition as when the rifle shot well, then I would consider sighting equipment. Make sure the base and rings are tight and firmly attached to the scope. It should be nearly impossible to move or twist the scope in the rings by hand without loosening them first.
If everything else has been eliminated, then I’d consider the scope itself the prime suspect. You can replace the suspect scope with a known good scope, or reverse the process and put the suspect scope on known good rifle. That should tell you whether or not the scope is the problem.
Good luck.
