Batteries normally deteriorate slowly with time. If you left it for a while, and when you switched it back on, there was a problem, it can normally be brought back to life. It's the same with any sudden problem. The cells don't break, so if something happened suddenly, it must be a wiring or connection problem, in which case it depends on your skill and/or resolve to fix it. You need a minimum of a screwdriver and voltmeter to check it out.