LiveSuite and ADB do not play nice together. It's why in the rooting instruction sticky I posted in red letters to not do what you attempted to do. In your case, all partitions in the internal memory save the recovery partition have been completely blanked out. Worse, they're most likely marked as "read only", meaning there's no way to access them. The black screen is what results when trying to boot to a partition with nothing on it.
Your options are these:
1. Return to the place where you bought it and exchange it for a new one, then follow the rooting instructions given in the link I provide in my signature.
2. Sell it on ebay.
3. Send it to me.
Coby is out of business so you have no warranty whatsoever.
There is always a risk when modifying a tablet something could go wrong, and you could brick you tablet, you assume that risk, your are responsible if you break your tablet.
I have put together a stock rom and a custom rom for 9742, these rom's are based off of Traveller1701's dump.
The stock rom is as the name implies, the rom is stock not rooted.
Custom rom is based of my 8042 port of Mobiz_V.2 neither of these roms have been tested, they are both to be applied via cwm.
You need to install cwm and make a backup before trying either of these roms. You download one or both roms, your choice to your sdcard, computer or internal sdcard, if you download to computer you need to transfer to tablet, either sdcards should work.
These roms don't wipe /data or /cache, so to go completely stock, do a factory reset via cwm before applying stock rom. Same goes for custom rom if you want to start fresh, starting fresh is best but up to you.
These roms don't remove cwm recovery if you want to flash stock recovery, I can put one together, neither of these roms can be flashed via stock recovery though.
I don't have a 9742 to test, so I can't say how well they work or don't work, I have included Traveller1701's boot.img all the bottom of this thread, seeing these roms are made from his dump, his boot.img might perform the best with them, might not.
If something goes wrong, roms don't work as you want boot into recovery and restore you backup.
to get into recovery you can adb or download terminal from market.
custom rom Update 7/28/13This is now built from my grandson's 9742
Update 7/28/13 I have fixed the video problem and camera problem with 9742_Mobiz_V.2.zip, at the time I posted this rom, I didn't have a 9742, I now have access to one, so the new 9742_Mobiz_V.2.zip is built from my grandson's 9742.
Note 9742_Mobiz_V.2.zip on first bootup takes a while to get up to speed, by default it scans internal and external sdcards for images. I recommend installing SuperSU from market, then install No-frills CPU Control.
adjust min to 912
change performance to fantasy.
If you get no touch screen reboot to recovery then flash 9742_modules.zip