Oh, I thought I 've lost it but it came back suddenly this morning! No refreshing or hacking is needed.(just by a single touch by my girlfriend!lol) I don't know what went wrong but it seems temporary...
for those who happen to encounter problems like this: don't panic, just try again sometime and it may be ok
to ericshliao: yes, it is a Chinese version. I have no confidence in the dealer since the warranty is already expired and iliad is a rather old toy of iRex...still, THANKS FOR YOUR ADVICE!