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,
Shawn