When you open a p:IGI+ project, this locks the project (using a hidden lock file) so that other users cannot edit the same project as you are using. This lock file is removed when you close a project (or exit p:IGI+) however if p:IGI+ does not close the project normally, or is killed for some reason, this lock file will remain in the folder.


To open the file again it is necessary to rename the file to a different name, however sometimes it is more useful to remove the lock file. To do this, in the windows search bar type "show hidden files" - and select the "Show hidden files and folders" action which is part of Control Panel for Windows. This is bring up a File Explorer Options dialogue:


Make sure you select to show hidden files and folders, but also uncheck the "Hide protected operating system files (Recommended)". You will then see a file with the name "~myFile.pigi" if the project you were using is called "myFile.pigi", the ~ denoting this to be hidden file. 


If you delete this hidden file it will then enable you to open your project again. If you wish, you can rename the hidden file (this can be useful to recover a project with changes where you were not able to save) to something like "myNewFile.pigi" (make sure to remove the ~) and you should then be able to open this project in p:IGI+ too.