I had an experience with exactly the same problem in High School 30+ years ago. My best friend was given a very ratty Arisaka as payment for mowing a lawn. It had the same problem you describe. We borrowed one of my Dad's Arisakas to diagnose the cause. Naturally, we disassembled my dad's rifle and succeeded in duplicating the problem and locking it up as well!
The problem (IIRC) is that whoever disassembled the bolt on your rifle reassembled it with the safety knob installed "upside down" (or rotated 180 degrees from correct installation). If you remove the action from the stock and look (again, IIRC) on the left side of the action (opposite the bolt handle) there's a slot giving access to a spring-loaded "tooth" or detente that locks the bolt in place (like a bolt in a door lock engaging the door frame). If you use a thin screwdriver to push that tooth back far enough to allow you to rotate the bolt you can then extract the bolt, disassemble it and then reassemble it correctly.
We used that method to disassemble both my dad's rifle and my friend's rifle as well. It's still sitting in my rifle cabinet in the same condition from that day, fwiw...
Last edited by lescoulee; September 8, 2009 at 02:30 PM.