Lots of reasons, why something does not work anymore. Most likely is a bad or even dead battery, so you can't charge it anymore. Sometimes only the battery contacts are dirty or oxidised. The same can be wrong with all contacts in the device. Dirty, oxidised, or not strong enough to make proper contact. As an electronics engineer, I think it is impossible to tell what is the real culprit what not having the device on my work bench. My advice is yo visit a phone (and tablet) repair shop and ask them what it will cost to diagnose the tablet and what it will cost to repair it.
Perhaps you even start saving up for a new tablet? they perform more for less money and you get a year of warranty if you buy a new one.