When you password-protect an iGrafx file (document) with the Tools menu Protect Document command, on subsequent open of the file, it cannot be edited until you use the Tools menu Unprotect Document command and enter the correct password.
If you have forgotten (lost) the password to the document, there is no way for you to unprotect the document, and you will no longer be able to edit it. You can, however, still open the document and see a read-only copy of the data. You can copy and paste this data to a new file for editing: Simply select (e.g. Select All with a Ctrl+A key combination), copy (Ctrl+C), and paste (Ctrl+V) the contents of each diagram (or other component) from the document to another document, and then edit that copy of the document. You can, of course, also re-create the contents of the document if desired.