Somehow all of my LibreOffice work got converted to Microsoft Excel. How can I reconvert them all back to LibreOffice
Strange. Something like converting all files to a different format doesn’t happen by itself. Depending on the number of files, you could open all of them and manually save them as OpenDocument Format files, or try How can I convert Word .doc files into .odt files on a MAC or see Batch conversion with unoconv in Windows
They didn’t. You probably reinstalled MS Office or somehow answered yes to a question if you want Excel always to open your .ods documents. To change it back, see the last comment on this question by @john.bandaru : LibreOffice doesn't recognize extension .ods