Aura HD Screen frozen, can't connect to PC via USB

My Kobo Aura HD worked well in the moring. But after lunch, it was frozen on the "Kobo Start Menu". The last action I did on it before lunch was select "power off".

After lunch, I slided the power button, but nothing happen.

then, I force turn it off (slide power button 10s), but the screen didn't change.
turn on again, screen still frozen,

force power off, hold light button and power button, get in hard factory reset, the LED blinked rapidly, but the screen still frozon...

when LED off, plug in usb, nothing happend in both linux(Ubuntu)and windows7.

plug out usb and then in again, LED blinked in normal speed, screen still frozen. If push the light button, the backlight on, from dark to bright; it will off when push the button again. But it still can not connect to PC.

after several times force power off, reset, turn on, suddenly, it can connect to win7, then I use Kobo deskop to register and upgrade it, hope this can solve the problem.
Unfortunately, I am wrong, screen still frozen, the worse thing is that I can not make it connect to PC again...

Any idea what is wrong with my reader?
Any suggestion will be appreciated.

Sorry for my poor English, hope I explain my problem clear enough...