I have a Windows 10 which I got thru exercise installer. Every time I export, the first 15 or twenty minutes have disappeared
What is an “exercise installer”?
What version of Audacity do you have? Give us all three numbers, see the pink panel at the top of the page. Use 2.1.2 from http://audacityteam.org/download/windows because in that version Audacity won’t let you set a temporary directory that is prone to cleanup by security and optimisation applications.
Also if you have a selection in the blue waves but want to export all, use File > Export Audio… not File > Export Selected Audio… .
Gale