It reverted to older version of the file and undi all my changes.
PC win 10 with all the latest drivers, new computer.
Impossible to say anything. You have not provided any explanation about what you actually did, the specifics of the file, how and where you saved it and so on.
Simply working on a muti page file. Open , make changes, save it. Now when I open it again, it revert to the previous unsaved version.
I have googled that this happened in previous version, not sure if it is happening again.
This is often caused by the setting "Save in background" in the Illustrator preferences.