Unfortunately, you are going to have to reset the tablet back to factory default. This will erase all of the data that has been added to it since it was first purchased. All photos, movies, apps, personal info. With that said power off the tablet and follow the below procedure.
1. take a look at the bottom left most portion of the tablet2. you will see a small pin sized hole (image of where to find it http://www.hard-reset.com/acer-iconia-tab-a500-hard-reset.html )3. locate a paper clip, and gently press it into the hole (ensure) your device is powered on before you proceed. 4. press and hold for 10 seconds, then release5. your device will now be reset back to factory defaults.
SORRY BUT I TRIED THAT ALREADY DIDNT DO ANYTHING
ok, i have some rather unfortunate news. the only way to perform an actual factory reset is through the software interface (which is locked on your device) the only way to unlock it is to send it back to Acer for service. The below is a link from the ACER support site itself explaining that this is the only resolution.
I'm sorry it's not something you can do on your own and will need to chat with acer to schedule the service return. below is the link to ACER support chat
I hope i have been able to answer our question and i assure you this is the only way to get your deivce working again.