Aura HD infinite loop reboot

Hi, I've tryed to install koreader (old version) on my kobo aura hd (with kobo launcher installed)... but now it goes in an infinite loop restarting... I've pressed the reset button for ten secondo also, but it doesn't do nothing...
the reader shows the string "kobo" and than the 5 cube loading...

even if it is plug in into pc it doesn't do nothing...How can I do????