1) shut it down completely by pressing the power button for 10-15 seconds.

2) connect it to your PC by USB

3) start the RKBatchtool or RKAndroidTool

4) put it in download mode by keeping VOL+ pressed, pressing power for 5 seconds, releasing power and then releasing VOL+. You should hear the USB connection sound on windows and the RK tool should indicate an RK Loader connection

5) Flash it with a known good ROM, like this one: http://crewrktablets.arctablet.com/?p=3319 (works fine on my Cube U9GT5) or GlassRomV5

NB: Getting RKBatchTool or RKAndroidTool to work is a challenge by itself, especially on Windows 8, but I suppose you managed to do it since you bricked your tablet!