Comment 41 for bug 264462

radeontool in Jaunty* can be used to assist in debugging this issue. After installing it, you run it like this:
    radeontool regmatch '*' > regdump_good.txt
    radeontool regmatch '*' > regdump_broke.txt

Run it two times. Once when you have a working screen (for any driver), and once in the broken case (either from the tty console or logged into the sick box remotely). Attach both of those to this bug report, and we can then forward this issue upstream. Thanks ahead of time!

For more info see https://wiki.ubuntu.com/X/Troubleshooting/BlankScreen

[* Note: radeontool is available in earlier versions of Ubuntu but is too old; if you need to run it on an older version of Ubuntu, you can obtain and build it from the upstream git tree at http://cgit.freedesktop.org/~airlied/radeontool/]