Well, your battery could be dead, your charger cord could be damaged, or your charging port could be.
The simplest one to check is the charger cord. Try your cord on another device, and try another cord on yours. If yours works fine on another device, it isn't the problem. If the other devices cord still won't charge yours, but there is no 'wiggle' in the charging port, then I would be looking at the battery needing replacement.
If your tablet is under warranty, then you should contact the manufacturer and they can repair or replace it for you.