The only thing I can think of is that maybe you checked it out on a different computer. A check out is specific to a user and a computer. If one of those things changes you do not have access to it. If you checked it out on a computer you don't have access to any more you will have to undo checkout and then check it out again.
If the computer and username are the same you will have to attach some screenshots of your PDM permissions and settings so that we can see if we notice anything that might cause it.
I dont know what causes it. I used the files on this very same computer, but when I checked it in and then back out it stopped. This have happened few times, if I have time I will try to take closer look what might cause it.
I do not know as in this case, but in case of such strangenesses I first of all check settings of the menu of users.
I cannot tell why, sometimes settings for not clear reason are reset (become empty).
After their reset the unclear problem usually disappears.
I cannot approve anything - just interestingly - there are no problems in settings of the menu (especially a file choice) for groups, the user or all users?