Depends on the tank and how good you are with silicone and glass work, and, of course, on the crack.
Small chips in the edges can be easy to save with some silicone, but you'd probably want to test the tank full in the garden before the living room.
A crack in the main panel will need a new panel ideally, which can be a real pig to do and may not be worth it with many tanks.
A tank with curved edges with a crack in one side is basically good for recycling.
A lot of the time it's more sensible to just buy another tank than it is to risk your fish and your living room.