While booting display blinks twice and nothing happened.It seems that Xubuntu started but display shows nothing.Also when if i chose previous Linux versions or repair option in GRUB it boots normally.
Without more details it's hard to suggest anything but one thing you can do if you believe the system has already booted is switching to the first terminal by pressing
CTRL+ALT+F1 (to F6 if I remember correctly). CTRL is needed for when the xserver has already started.
Then you can start looking at log files if your system is already up.