Try doing another Factory Reset but this time try it again by making sure your Venue 8 is powered off. Press and hold the Power button for ten seconds or so to force it off. Now set it aside and charge it up completely. Start it up again into its Recovery Mode (I believe Dell refers to this a DroidBoot) and before you do the Factory Reset, wipe the cache partition first and then select Factory Reset.
If this doesn't work and it still isn't booting up properly, you might have to go through the process of reflashing the stock ROM. Note that a Factory Reset only wipes the general user data partition, where your personal files and things are stored. It does NOT do anything to the actual Android operating system, which resides in its own separate, protected partitions. Reflashing the stock ROM is basically reloading the operating system. a different process than a Factory Reset. It's also a bit more involved. Here's a link that runs you through the process:
http://downloadstockrom.com/stock-rom-for-dell-venue-8.html