Hello Abhishek,
When this problem occurs, I cannot even launch enpass from the tray icons, so I do not believe step 3 (cache and cookies) is related. To view enpass again when this happens, I need to launch it from the Start menu.
I am running 6.8.6 of enpass, the latest Firefox (but, again, I can't even launch it from the tray icons...)
Sorry it took me so long to reply, I forgot to check back.