It seems (sadly) that the built-in e1000e driver for an Intel Gigabit Ethernet controller does not work in Ubuntu 15.04. I found a workaround by obtaining a USB Gigabit Ethernet Adapter, but I wonder why the current Intel drivers are not available in the kernel?
I had a double-whammy of both the Nvidia grub mode issue (had to set the nomodeset option in Grub) and the e1000e driver not working in 15.04 (no networking), but I managed to get a USB apapter to work and the machine online though not with the onboard ethernet controller.
It made it really hard to have both of these issues occur at once. I've been using Ubuntu since Dapper days and this was the first time two issues hit me at once. it was hard to report anything when you can't see the screen and are off the network at the same time.
I would install the Intel driver, but I want to know if I have to recompile it for every kernel update? If so, I will stick with a working ethernet adapter and be done with the issue.
Have a nice day!