Beaglebone black does not enumerate as ttyUSB0 in Ubuntu 14.04

My setup :

Ubuntu 14.04 LTS dual boot with windows 7.

I connected usb power cable which is a 5 volt supply. I also connected a usb to uart serial cable, and was able to see this to be as ttyUSB0 in ubuntu few days back.

After I did not see this ttyUSB0 only ttyACM0 (which I think is from usb power cable).

I have one doubt what is overcurrent?

I checked what power usb to uart cable is getting and it was 5 volts. I checked the voltage on serial header of beaglebone and it was 3.3 volts. How ever I am not connecting red cable on beaglebone black.

I am unable to understand what exactly is the problem?

Please respond anything related to it.