I installed Ubuntu 12.04 by replacing my Windows installation. Everything worked perfectly. Then it asked me to restart. I did, but it does not go into a selection menu or start up menu. The screen is black and shows a blinking white stripe in the left top corner. Even after waiting 1 hour it did not start anything. What am I doing wrong?
If it is a new machine/laptop/netbook, then it may have "Secure Boot" switched on in the BIOS. In this case it may help to switch "Secure Boot" with UEFI to off in the BIOS. You may also try Boot Repair