this is all i could find out.
When the tablet is turned off, keep home button pressed.
Now turn on tablet while still holding the home button.
When white android figure appears, release both buttons.
You will get into recovery mode, you can select "restore factory settings" there.
Download the right firmware here: ONDA
Extract the content to the root of your sd card (since you cannot use your tablet, i suggest using a cardreader + pc or another tablet? or try to hook your tablet to the pc and hope it recognizes it)
There should be 5-6 files now on your SD card.
Turn on tablet while holding home button, untill white android figure appears then release.
It should update your firmware now...
or you could try this link on how to do a hard reset.
but do so at your own risk.
Onda V811 hard reset format factory mod recovery rom install
or try this link for more info.
hard reset tablet onda v811 Google Search
or this one.
811 Onda Tablet logo screen white Google Search