Strange Headspace Problem

The gun is a semi custom (stock barrel) build with a long Accutrigger Action that was trued and timed. It is chambered in 7mm WSM. The chamber has always been tight and all reloads have to be full length resized. With approximately 250 rounds on the gun, I was at the range shooting when suddenly I could no longer chamber a round (the bolt handle would not close all the way). When I got home I examined the gun carefully and tried to chamber a Go Gauge, the bolt would not close on the Go gauge. It would close on the gauge previously.

I have cleaned the gun, making sure there is no carbon build up in chamber. I have disassembled and cleaned the bolt. The only thing I did not disassemble on the bolt is the the spring loaded pin on the bolt face, but it seems to be functioning properly. The bolt still will not close on the Go gauge. The barrel nut does not appear to be loose and I can’t imagine how the headspace could have been decreased. Does anyone have any ideas what could be going on?
Thanks for your help.

Are you saving your brass? If not did you look at it? You could have had a piece come into and you have the shoulder and neck still in the barrel. Also get hold of the barrel with one hand and action with the other and try to unscrew the barrel. Any give would show the possibility of barrel movement causing headspace to change. The other most likely thing is ammo just a bit out of spec for the chamber which seems unlikely since the headspace gauge did work and now does not.

I would look at the front action screw to make sure it isn't sticking to far into the action. If it is, this could cause the bolt not travel full rotation.

Did you happen to tighten your front rec screw?Did you check to see if your ejector was free and clean? Any pierced primers? Dirt , debris in locking lugs? How about a case that left its neck in the chamber, Just because the locking nut seems tight doesnt mean the barrel cant rotate ,making the headspace shorter, ive seen it happen with fast twist , large calib ,hot loaded chamberings

Whoops........ Thanks Sean that should have been first on my list. Old age brain gas.

Thank you so much for the great responses!!
It was the forward action screw. As soon as I backed it out, all was well. It had been trimmed already prior to installation, but it worked it's way in a tiny bit more and blocked the bolt head from rotating. I'll trim it some more...
thanks again,
Unflappable