how to unlock a locked file

I have never had this before...I have a locked file and I cannot figure out how to unlock it. I am an avid Photoshop user. To day I was editing a picture and when I went to save it, message is locked go to explorer to unlock. I did and I cannot figure what to do. I have about 12 files in a file, only one is locked, all of the rest I can edit and save with no problem. The pic are on a card, I put card into down loader and put them into a new folder for them...and I start the editing procedure...edit and save. until I get this new message on just on file(image) and i cannot figure out how to unlock it.(it is driver me crazy) I could delete it and go on with life and but it is bugging me because I like the picture.
So in simple terms and step by step how do I correct this situation
The pc is a dell with win 7 ultimate on it
Kas

Thanks for the shout, I did what you said...it did not work!!! I went into the file from the camera and look at the properties of the file and I found the both the read and archive box's were check, then I looked at the file of the same group, and looked their properties were just check for archive. So then went to the problem file and checked just one (archive) and started the editing process and voila it worked!!!! somehow both got checked....well the problem got solved and thank you for getting me to look before the file got process by the pc.

Kas:):):):)

Glad I could be of service, now we can put my problem to bed and call it solve
Thank you for the help
Kas