Some things it could be:
-The battery (used up or defective)
-The display (liquid damage, injury or failure)
-Liquid Damage (user error)
-The port (pin damage, liquid damage, burn)
-The USB cable (damage or failure)
-The USB port on your computer (needs software update if PC, or may have incurred damage, liquid or otherwise to the logic board.)
-The power adapter (failure, generally)
-Plug into your power adapter. Leave connected for ten minutes. You'll want to test for ten minutes since the battery may be so dead it requires enough time to charge to power on. If after ten minutes, there is no sign of life, try connecting same power adapter to different mobile device (except for the iPad, that requires a 10W adapter) If you have a 10W adapter, it is safe to test with your iPod and in fact will charge the device faster than a 5W.
If it charges other units but not yours, the issue stems from the device itself.
If the unit does not charge any device, it is either the cable or the adapter itself. Test with Known Good cable first and then with KG adapter.
-If the power adapter and cable will charge other things, while leaving it plugged in for ten minutes, try a hard reset by holding down the sleep/wake and home buttons till you see the apple come on.
If that doesn't work proceed with the next step.
-If you're not tied down to the data on your iPod touch, you can try restoring the software and see if that works.
-Launch iTunes and plug in your iPod .
-Attach the phone to your Mac or PC.
-Hold sleep/wake and home together for around 10 seconds
-Release power but keep holding home until iTunes recognizes a USB device is plugged in.
-At no point will the display come on. If it does, start over from the hold sleep/wake button and home step.
if iTunes will recognize the device and tell you there is a device in recovery mode then a restore is necessary. When you click restore, all of your information will be gone. I repeat, ALL OF IT. The interface will then install Known Good software and you should be able to use the unit normally.
If you get an error message during this process or if none of these methods work for you, please make an appt at your local apple store for them to troubleshoot your issue. The session is free and if you're covered under warranty and have no sign of liquid or accidental damage, they will replace it for free.
Hope this helps