Hello Ionut, and welcome to Seven Forums.
That is strange. You might see if booting into the Advanced Boot Options
screen (tapping F8 during boot), then selecting either Safe Mode
or Enable Low Resolution Video (640 X 480)
may allow you to boot into Windows 7 to check your screen resolution
and refresh rate
settings to make sure that they are correct for you monitor. While there you may also need to uninstall and/or install your video driver.
Hope this helps,