As FarmHand said above, you may have a slow drain like a radio clock that may have discharged the battery , there are lots of things that could be wrong, it would help to know what tractor you have, be sure the cable ends and post are silver color and not coated in lead oxidation (gray color) as power will not pass through it, sometimes it takes more that a wire brush battery cleaner to get it off, if you have the replacement cable ends, they can give issues where the wire is clamped to the end, some have clutch switches, others have neutral switches, start relays. rat damage ect.