How do I unlock a read-only Word document form?
Open your document in Word, then use "Save as..." to save it in ".xml" format.
Open the .xml file in wordpad, notepad, emacs, or other text editor of your choice.
Search for the string w:enforcement="1" (or w:enforcement="on").
Replace the "1" with a "0" (or replace "on" with "off") to disable enforcement. (This step unlocks the document.)
Save the .xml document from your text editor.
Open the .xml document in Word.
Choose "Save as..." and save it as a .doc or .docx file.
Your original Word document can now be edited normally. Enjoy!
As you said it sounds like the file is locked. To open it you need to went to Developer > Protect Document, like you do, but then to enter to "Restrict Formatting and Editing". Now click Stop Protection at the bottom of the task pane and enter the password (if you know it, if the "Stop Protection" doesn't appear the file is not locked).
If the file is locked and you don't know the password you can try to use this guide to unlock it (disclaimer: I don't try this guide and I don't know if it works or not (but according to the comments in forum where I found it (with a brief search on Google) he is doing the job)).
If the "Stop Protection" doesn't appear and you still can't edit, comment here and I'll try to help more.