Did you download and install fglrx from the .run installer from AMD? Even if you have done this in the past and then installed fglrx from the repos, it will still break the free radeon driver.
The uninstall script is usually in /usr/share/ati/
# aptitude purge ~nfglrx
# sh /usr/share/ati/fglrx-uninstall.sh
# aptitude reinstall libgl1-mesa-dri libdrm-radeon1 libdrm2 xserver-xorg-core xserver-xorg-video-ati xserver-xorg-video-radeon
Get rid of xorg.conf (also check for fglrx related conf files in /usr/share/X11/xorg.conf.d/ and /etc/X11/xorg.conf.d/
# mv /etc/X11/xorg.conf /etc/X11/xorg.old
Then ensure that radeon is not blacklisted. Check for a blacklist file specific to radeon in /etc/modprobe.d/ and also open /etc/modprobe.d/blacklist.conf and ensure that radeon is not blacklist there.
Next ensure that KMS is enabled, either edit the file /etc/modprobe.d/radeon-kms.conf and ensure that is says "options radeon modeset=1
Or you can just recreate it as follows
# echo options radeon modeset=1 > /etc/modprobe.d/radeon-kms.conf
Install the non-free firmware package (contains the radeon microcode)
# aptitude install firmware-linux-nonfree