Lens won't focus at infinity

I just acquired a 300mm lens for my Pentax 67 kit. It looks like new, yet it won't focus at infinity. I was out in the Alabama Hills (in CA) and my subjects were certainly far enough away to require the infinity setting. Perhaps it was dropped in transit, and while in it's nice hard case, sustained some misalignment internally. It shows no external damage or even any signs of use. Is this easy to repair?

I just returned a camera like that. It could be a problem with the lens, the mirror or the viewfinder. Very tough to track down without a spare lens, body and finder to narrow the possibilities.

The focusing barrel may have been loosened at one time allowing it to shift postion slightly. Or improperly assembled etc.
There is usually an internal infinity stop which is adjustable, or the focusing ring itself has the stop machined into it, in which case it may have slipped.
To check the body lens combo use a small GG at the film plane to see if you get sharp focus. Do the same with another lens to verify if probelm is body(unlikely) or lens(MOST likely).

Thanks, guys. I figured out the problem and fixed it. This lens was in storage for a long time and the focusing ring turns against a lot of resistance. Somehow this caused the front section of the lens, with it's optics, to unscrew a bit, knocking the focusing out of wack. I finally saw that, and simply tightened that down by hand. Focus is now fine!