The startup pop-up window cannot log in to your account.

There were no issues with the computer previously. Today, when I booted it up, it displayed a message stating that I couldn't log in to my account. Some of the files on the computer were missing. (It seems like they weren't actually lost.) For example, there were originally 30 software programs on the desktop, but now there are only 15 left. The disk memory usage percentage is normal. It should be because there is no account, so there are some usage restrictions. However, the items in the user folder and the files on the desktop are all missing. Now, data is not being saved. After each restart, any usage records from the previous session disappear, and it reverts to this state where the account cannot be logged into. How can this be resolved?