Maybe you can give us a little more information about your system. Is it a computer? A mobile device? Running Windows or Mac OS? How long between the time you install it and when you tried to run it again? Anything happen in between, like a Operating System upgrade?
Any, all that info would help better determine why you’re having issues.
As for working offline, yes absolutely! That is one of the key design features of Mylio, that it maintains your entire library in your devices so that you can always access it and work offline.