I had this exact issue on my machine where I am running both Fedora 20 x64 and Windows 8.1. Sometime in the summer I changed the GPU (Gigabyte GeForce GTX 750 Ti) to support two monitors both using the HDMI interface. Today I needed to switch onto Fedora and was surprised by the same fuzzy (corrupted) screen on both monitors (LG 22M45).
I did the following (below I list two links where I found the procedure):
Ctrl + Alt + F4 at the fuzzy screen in order to bring up a terminal which was displayed correctly.
Then I logged on and performed:
sudo yum update
Afterwards I did a reboot.
Installed the nvidia drivers (the author of the solution recommends akmod instead of
sudo yum install kmod-nvidia xorg-x11-drv-nvidia-libs
Finally, I removed Nouveau from
initramfs by performing:
mv /boot/initramfs-$(uname -r).img /boot/initramfs-$(uname -r)-nouveau.img
dracut /boot/initramfs-$(uname -r).img $(uname -r)
Then, I did a reboot and the next time the Fedora logon screen appeared correctly.
Here is the original procedure. The same instructions, but more concise, are also here.