Hyper-V Ubuntu Server 16.04 stopped working

I am ruing Ubuntu Server through Hyper-V on my desktop Dell. My hard drive is on a WD My Passport Ultra 2 TB exteal Hard drive. I used the generation 2 configuration on Hyper-V. I've had it working for about 2 months. Sunday, my server was up and ruing fine. Yesterday, I tried to tu it on and it coected, it says start was successful and that it is ruing but the screen is black. I tried restarting my computer 5 times, but it didn't help. So, I installed a new virtual machine, the same way, Hyper-V with the hard drive on my exteal hard drive. It worked fine yesterday, but today the same thing is happening to this one. Does anyone know what the problem is? Could my exteal hard drive be the problem? Thanks for you help.

I also have Ubuntu Desktop ruing through Hyper-v with the hard drive on my WD Passport, and it continues to work without any problems.