If you have tried to do a "Hard Reset" (which is why one would push those buttons together) and the phone won't start, it may just be without any charge.
Have you plugged it in? Note, if it is at 0% battery, it will need to charge for a few hours before you can attempt to use it.
You should also make sure there is nothing in the plug (the part that goes into the device, on the cord) or in the devices port. Dust, lint, dirt, etc., can get into it and then it can cause you not to get a connection that will work right.
There could also be a problem with the charger, the charger port and/or the battery itself.
First you need to make sure the charger cord is working. You will need to try a different cord or try using that cord you have in a different device.
Should that not be the problem, you would need to then see if it is the battery. Again, by trying a different one.
If both the battery and cord are working fine, check to see if the port is loose. When you put in the charger cord does it wiggle? If so the port may be loose or damaged. If this is the case it would need to be looked at (unless you feel comfortable doing the work yourself) and probably replaced. Which would (in most cases) include changing out the motherboard as well, as the port is attached to it.
If none of this is the issue, then you may well need to replace the tablet.