I am using Kubuntu 14.04 for more than a year without any problems. Today I booted to a Live USB Kubuntu 16.04 to see what's new and so decide to meke the upgrade or not. I just check if all the hardware was correctly detected and was working as expected. Then took off the USB and reboot the machine... To my surprise Kubuntu is no longer starting. Initial boot seems to be working and Kubunto logo appears for a few second, then the screen turns black. I could hear the hard disk seeking for 30 or 40 seconds, and then the system hangs. Tried to boot to recovery mode, and the disk and data are fine. If I tried to resume I get a text login; I can switch with Ctrl+Alt+F1,F2,F3 back and forth but the graphical desktop never shows.
Please can anyone tell me how to recover my system, or tell me what's going on.