Your video settings are probably screwed up.
It happened to me too with my Radeon 9250.
Your best option is to use the Fedora core 6 installation disk to boot.
Once the CD or DVD has booted, then select the safe mode option (or something like that).
It will ask you if you wish to start your network, but you can answer no to that. And whn you finnaly get to the command prompt, then type in this command:
Then find a section in that file that looks like this:
The driver might not be the same as in my case, but it should look like that.
First, the "safest" video driver to try is the vesa driver. You can't really go wrong with it. It doesn't provide any spectacular 3d performance, but it will get you up and running, and you can worry about extras later.
So change where it says Driver "radeon"
(ar whatever you have in your file) to Driver "vesa"
Next, search through the file and if you find any lines refering to the monitors refresh rates then comment them out (by puttig a hash # before the line).
They look like this:
If you do find them then comment them out so they look like this:
Now save the file by pressing CTRL+O and then exit the test editor with CTRL+X
and hit enter (probably best take out the CD or DVD),a dn wait for the computer to reboot.
If it works, then we can move on to getting full hardware acceleration working