You probably need to install drivers for your graphics card. I can give you a step by step if it was Nvidia.
For ATI I can only point.
You need to boot into single user mode to install graphics drivers (providing you have an internet connection), in Fedora 7 I hit any key at the boot splash, type a
to edit kernel boot parameters. At the end of the kernel parameter line is the word "quiet", add a space and type the number 1
and hit enter.
This should boot into command line only and ask for admin. password. Once logged in as root try the commands suggested here
to enable rpmfusion repository (under "Requirements"), and install drivers.
Then issue command: reboot
to reboot into "hopefully" graphics mode.