All my characters have become question marks
You do not specify operating system, language, keyboard nor version of LibreOffice which is necessary to try to understand your problem.
Do you mean � U+fffd which is the REPLACEMENT CHARACTER used to replace an incoming character whose value is unknown or unrepresentable in Unicode? This is sometimes found when cutting and pasting text on a system not running native Unicode, for example Windows or Apple, or sending an email. The operating system system does not recognise the Unicode character.
If it is an  (U+fffc), this is found when a genuine Unicode character is written, but the current font in use does not contain a glyph for this character.
This does not answer your problem but might provide some help…