The reason why the battery sign has a X on it is because you are using the wrong charger. When you use the wrong charger, it could cause problems in the tablet. Some chargers are strong than others and some chargers are weaker than others. If you use a charger that is stronger, the tablet would get way to hot. The tablet will probably not turn on if you keep doing it. (The tablet should tell you that it's to hot.)
TIP: If it has the X on the battery sign, turn the tablet off and don't mess with it until it's charged completely. (If you play on the tablet while it has the X on on the battery sign, then it won't charge.)
If you are using the right charger, then there might either be something wrong with the charge or the tablet. If there's something wrong with the tablet, there might be something loose in the tablet, due to dropping it. If there something wrong with the charger, check the charger cable. Look for any visible wires. If there is visible wires get a new charger. (If there isn't any visible wires, I really can't help you. You should take it to the shop.) One reason why you might see visible wires is because you probably kept bending the charger. When you do that, the charger insulator will slowly rip.
Ex: If you keep folding paper on the same crease, the paper will paper will eventually rip without a problem.
Like the wire insulator
I've learned by experience.