You likely have the wrong graphics card driver. It could be the monitor, but
more like graphics card. You can check for a proprietary one by going to
Hardware drivers. If it is Ubuntu 9.04 and an Intel driver, you may have to
wait as Intel has been slow to produce drivers for the kernel used in 9.04.
If it is a previous version, then this is not an issue as it uses a
different kernel.

You can also open a terminal and type: lshw and look at the display section.
Post the results if you want.