I'm going to guess that there's a battery shutoff switch somewhere that's turned off. Often these are momentary toggle switches labeled something like "use" and "store", often located near the entry door or on a control panel with other switches and gauges. The actual shutoff is a remote operated latching relay that you can here click on or off when you press one side or the other of the switch.
Occasionally there is a shutoff that's a big manual switch located at or very close to the battery.
If that's not your problem, it sounds as though there's a bad or broken connection somewhere near the battery, or a blown main fuse. It may be the ground cable, of course, not just something on the positive side.