Default USB connection doosn't work

well well well,my device is a develpoer version with a unrelockable bootloader,suddenly I found this problem after I woke up days ago,I saw it appeared in my win7's explorer last night,but the next day it coundn't be connected anymore but only can be recharged.here're
the infomations I need your help
1.If my phone is already running,when it was connected,there was no notification in PC or the phoneif the phone was powered off and pluged with usb ,during the power on ,PC showed no notification but the phone showes USB Connected,but when it was unpluged,the USB notification was still therefastboot couldn't be connected either
2.I tried new USB cables and other PCs,and other roms,even I opened the phone and replaced the USB board,always the same
3.used Viper4.2 RC2 ROM&4.06 firmware&TWRP recovery that time
I want to know how it comes and how to solve it,in our Chinese forums I saw some guys trapped in the same satuation but there was no solution...
anybody knows the reason?