Why can't I open the file I recently closed?
Most likely, you accidentally changed the file extension for the file. Almost every file has an extension, 1 - 4 characters after the period in the file name that tells the computer what program opens that file. If those characters are missing, changed, or incorrect, the file probably won't open.
You might be able to rename the file with the correct extension, but it might not work, and the work will be lost. To determine the extension, look at the properties of another file that is similar to the one you're trying to correct. For example, Photoshop generally uses the extension PSD for the files it creates (but it can also save files in other formats, which have different extensions). Generally, capitalization is irrelevant in file names stored on a PC (It's often a different matter when dealing with servers).
Do not change the extension. Extended time on an assignment will not be given.