Sadly, quite nothing to do, see:
22 pages term paper replaced with pound signs.Any power shortage for example? It's a common root cause of such problem.
Check the temporary folder of the system (see in OOo Tools>Options>OOo>Paths). If there are folders like sgmlf.tmp with a file having the same name inside, make a copy of that file, rename it in .odt and cross your fingers. Else, I hope you had a backup (or a copy in a mailbox)...
It seems there are less troubles with latest version 3.4.1. What is your version exactly? Your forum signature shows only 3.
LibreOffice 7.0.3 on Xubuntu 20.10 and LibreOffice 6.4.6 (portable) on Windows 10.