I suggest trying the hardware reset if you haven't yet (pinhole, lower right corner near USB port).
You said you tried volume up and power, and I'm not certain if that means the Factory data reset described here: Acer Support: Frequently Asked Questions list for Internal Only - How do I perform a Factory Data Reset if my Acer ICONIA TAB A500 does not boot?
If the tablet is on -- e.g., stuck at the Android or Acer boot screen -- you must power down before starting the procedure by holding the power button for 5 seconds.
The button timing for the Factory data reset is very important. Hold the power button for only 3 secs only, continue holding volume up, and operate the screen rotation lock switch immediately after the vibration is felt.
Look at the upper left corner of the screen for:
Erasing user data...
If you don't see both messages, hold the power button for 5 seconds to power down again and start the procedure over. It can take a few tries to time it properly.