I didn't find a solution in the forum so thought this might help others.
Scenario: I was finalizing a Power BI Desktop report and while a report was rendering, an Unknown Error message appeared. All visualizations went blank with message "Something's wrong with one or more fields. See Details" I had made no changes but all reports were showing this error. I reported this through the send a frown option and it was said to be known. The last time I backed up the report was a week earlier so I had lost many hours of work. (lesson learned) I eventually noticed the Date table was gone as were any measures or columns I had added using Power BI. I have no idea how they disappeared or how I could recover these without a recent backup.
Solution: I found the most recent temp file and was able to restore a lot of my progress. The files are located at
\users\user\AppData\Local\Microsoft\Power BI Desktop\TempSaves
