does star crossed cancel if someone plays temporal breach on a target

No — Star-Crossed doesn't cancel. It still resolves and returns whichever of its two targets is still legal.

Temporal Breach banishes the unit (moving it to a non-board zone) and then replays it as a new game object, so that unit is no longer the object Star-Crossed chose and becomes an illegal target (Rule 359.3.e.4). But the other target — whichever of the friendly/enemy pair wasn't Breached — is still on the board, and per Rule 359.3.e.8 the spell executes "as much as possible," returning that unit to its owner's hand (FAQ #10821).

One note: because Temporal Breach's unit re-enters as a fresh object, it won't be the one returned to hand even though it's back at the same location.