Rooted my Asus a few days after I have received it. A few month later I unlocked the bootloader and here we go :) Made a backup of the original stock ROM with TWRP and using Goo.im I updated to latest CyanogenMod. Everything worked fine - until I restored the stock ROM using my backup.

Recovery worked fine - but after reboot to the new image I was stuck at the ASUS Splash Screen. Fastboot was working but the only options I had was USB or Wipe Data - no Android Symbol there and no Recovery.

I found this thread at XDA which unbricked my tablet. What you have to do is easy and straight forward, you have to install a new boot.blob - how?

• you need working ADB, fastboot and a ROM with a proper boot.blob (for example the latest Virtuous Rom)
• extract the ZIP file and pull the file called 'blob'
• rename the file as boot.blob and move it to the ADB TOOLS folder and open a Command Prompt
• boot your Prime into fastboot and select the USB symbol
• from inside the TOOLS folder type: fastboot -i 0x0b05 flash boot boot.blob
• then type: fastboot -i 0x0b05 reboot

This should grant you access to recovery again. From the recovery you are able to use ADB and can push a new ROM to your Prime.

• open a command prompt and type: adb push virtuous_prime_s_9.4.2.21_v1.zip /sdcard/
• wait until you get a confirmation message (it will look something like 1279 KB/s (334733438 bytes in 255.472s))
• flash ROM as you would do with any other ROM.

Thanks to the XDA member for solving this.