Updated to Parallels 10 and Yosemite today, when I open Windows 7 I can only see half the windows page and there is 2 screens with white lines. I've removed all Parallels from the computer and reloaded it and I'm still having the same issue. I've been at this for a few hours and most of my hair is on the desk around me.
I was able to lower the resolution about 6 screen sizes smaller by clicking all over the screen until dumb luck struck and I was able to blindly locate the ok button. Issue fixed.....going fix a strong drink now.
Ouch. I haven't experienced that particular situation. Do you recall what was the res on the Win7 VM under the previous version? I hope your hair grows back. Just for interest: With several linux VMs (I _think_ all XFCE or Lxde desktops) the res and extent on the monitor were wrong following install. First thing is always to install Parallels Tools, and then reboot the VM a couple of times from the host/parallels menu... and they have sorted themselves out.
This seems like a display driver issue, are Parallels Tools installed? You can restart (ctrl+alt+del two times if nothing else works). You can boot Windows into safe mode and perform the uninstall/reinstall of Parallels Tools. http://windows.microsoft.com/en-us/...-safe-mode#start-computer-safe-mode=windows-7