there's no subject though, so WHAT is useless is never specified. She could be saying "everything/life itself/etc. is useless without you".
"kimi ja nakya" could even be translated as "if he isn't you" or "if it isn't you". "it" could be interpreted a bunch of different ways here.
So it could be "if it isn't you i'm thinking of, everything is useless". Which would mean "I can't think of anything but you" is a somewhat accurate interpretation.
Or it could be "Anything that isn't you is useless."
There's alot about this line that's open-ended and up to the listener to interpret based on the context of the song & how they themselves feel.
All good poetry is like that, lol
