My students use LibreOffice and a password has been accidently set on a Document (writer). How do I get into that Document?
There are a number of password related Q&As on this site which you might want to browse. The most important for your problem (I think) is “How secure is LibreOffice when using password protect?” Good luck… 
I don’t know if you have a backup copy with an older, un-protected version of the file? Check the backup directory? You can find its location under Tools > Options > LibreOffice > Paths.