Where do you see the lock symbol? On windows explorer or LabVIEW project explorer? If it is in Windows explorer check whether the folder where the VI resides is shared. This generally happens lets say your PC has 2 different user access and from one user you are sharing the Folder/file, so for the other user it will appear to be locked which actually shared by the other user on the same PC. Or in other case it may be a read only folder/file.
Typically it happens if you have 2 projects open with some common VI's, in which case they'll be locked. Or you have some VI running, locking code.