I have just upgraded to Ubuntu 15.04 and get this on first and subsequent restarts "ACPI PCC probe failed, starting version 219" which just hangs my computer. I have to use recovery to get the computer to boot so I can send you this message. I am relatively new to Linux and would appreciate any help to resolve this issue. I have a dual boot with Windows 8.1 and don't want to upset anything that might mean a re-installation of both Windows and Ubuntu. Thanks in advance.