There are a couple of tricks to try: hold power for at least 10 secs, then release holding vol-down, use the small reset hole holding vol-down, power off then connect the charger while holding vol-down and possibly more. Several people managed to break out of such a boot loop using one or another of those.
Sent from my TF300T using Tapatalk
Thanks to Buster99
I have the same on my TF300t and A700 - but it seems i just managed to get my TF300t back
i tried all fastboot (reflashing recovery, boot, system etc) no dice - especially since writing system was done after 3 secs all the time.
Here how i restored my TF300T JB
download your appropriate lates firmware from Asus (WW, US etc.) and extract the zip twice so you have your blob file
install your device drivers if needed (i used Google SDK ones) (and as a Tip Win8 sucks for Fastboot stuff so try to get Win7 Vista XP for this)
Reset your device with a paperclip/needle (about 2 cm down of your sdcard slot) and hold vol-down to enter fastboot
(the folowing i did at least 10 times already but didn't erase misc)
fastboot erase system
fastboot erase recovery
fastboot erase userdata
fastboot erase boot
fastboot erase misc
fastboot erase cache
fastboot -i 0x0B05 flash system c:\adb\TF300t\blob (this is my path - your's may vary depending where you stored your blob) ((also this was the first time i saw the loading bar while flashing)*
fastboot -i 0x0B05 reboot
* it also took a lot longer as it should:
C:\>fastboot -i 0x0B05 flash system c:\adb\TF300t\blob
OKAY [ 1.695s]
sending 'system' (800935 KB)...
finished. total time: 288.789s
im now setting my tab up to get back to CM (which initially caused this sh1theap due to autoupdate error, same as my A700 got bricked )
hope i could help you out!