Well either your engine is not charging while it's running ( not likely) or something was left on, an interior or livewell light, a pump, it doesn't take much. If the battery is that dead it might be a lost cause, depends on low it got drained. You can use a jumpbox to trim it up, just hook it to your battery and while your at it look for what drained it dead. Some of those crestliner lights you tap the light to turn it on/off, the switch is in the light.