I got a similar issue.
I updated yesterday my 8.0 to 8.1 and it won't boot up anymore.
I tried to re-image a new SDCard, but it looks it is not working with my Rpi3B at all.
Tried my sd card on my sister's Rpi 3B, still not working, and also I tried with another SD card, no go.
How do i find a previous version to downgrade? I can only find the latest thru the media creator.
Thanks!