when i was in grub of the USB flash every time i chose try Ubuntu live or install Ubuntu the screen goes black i searched for solution and i found nomodeset option for it, i installed Ubuntu normally and rebooted to find the resolution dropped to 640x480 and the monitor are unknown in monitor preferences, here is the xrander read:
xrandr: Failed to get size of gamma for output default
Screen 0: minimum 640 x 480, current 640 x 480, maximum 640 x 480
default connected 640x480+0+0 0mm x 0mm
640x480 73.0*
i think that nomodeset option is the reason of this problem but if i turn it off, the boot screen will go black again!!!
Edited: my graphic card is radeon hd 5450 and the monitor is CRT from 2004
