If there's a chance there's a charge left, then you want to hold the power + home buttons pressed for at least 30 seconds to make sure there's isn't an app preventing it from shutting down, then throw the charger on it.
Thread to check out:
Touchpad won't charge or boot - HP Touchpad - RootzWiki
Otherwise, if it's completely dead and you can't resolve it by charging with the stock charger and stock cable or USB and you've checked the charger on a different device and you know it works, then the only recourse may be to open it up and charge the battery directly (best left to an expert).
The stock charge is rated at 2 amps so you really need to be using a charger rated at 2 amps.