Did you really name it index.xar?
Is it possible you called it something else?
Would you have changed its name at some point?
Have you used Windows Search for any and all .XAR files?

Failing all the above, have you checked File > Open recent > Backups... for the last version saved?
It will be index_<some_date_some_time>.web, usually sitting under L:\Users\<YOU>\AppData\Local\Xara\WebDesignerPremium\9.0\Backups or L:\Users\<YOU>\AppData\Local\Xara\XtremePro\9.2\Backups.

Acorn