Today I was reviewing my old .psd drawings, and to my regret, I learned that one of them opens with a strange mistake. The file has an unknown format. I can’t believe this! Before everything worked and opened without any problems.

This one article from Adobe, is also can assist you repair psd file: https://helpx.adobe.com/photoshop/kb/file-recovery-photoshop.html or
https://onlinefilerepair.com/en/photoshop-repair-online.html

· Open "My Computer." In Windows XP, the icon is on the desktop. In Windows Vista or Windows 7, you must access this via the "Computer" option in the Start menu.
· Double-click the "C" drive, followed by the "Windows" folder inside. Locate the temporary Photoshop folder, which will look similar to this: "Photoshop Temp6382948398." Double-click the folder when you find it.
· Locate the file inside the temporary folder that matches the name of the corrupted file. Double-click it to open it in Photoshop.
· Click "Save As" in the "File" portion of the main menu bar. Save it in a different location and under a different name.