Question mark on Macbook startup - lack of disk space?

Unfortunately, it sounds like your data may be gone, but you could try reinstalling macOS first before reformatting - How to reinstall macOS - Apple Support

If your Mac starts up to a question mark - Apple Support

You can- but you'd need to know the exact name of the file. If you do, you can run this command (replacing 'filename.pdf' with the correct file name and extension-

rm filename.pdf

If you have spaces in the name, you have to use quotes around it-

rm "file name.pdf"

Note that both these methods irrecoverably delete the files in question, so be sure.

You can also delete a user profile, but you'll have to know the name of the account, not the user. For example, the name may be "Johnny Appleseed", but the user folder may be named something like "johnnyapp" (it'll be whatever the user put in). To delete a user profile, simply use this command, replacing "username" with the actual one.

sudo dscl. -delete /Users/"username"