If you use a USB display adapter to add a 2nd screen to your Mac, you may have noticed that ever since Mac OS X 10.5.7 came out, they have been mysteriously failing. This morning, after logging out and logging back in, the black-screen-of-doom scenario happened on my Mac mini's second monitor. By searching around, I came across a DisplayLink FAQ that seemed to provide answers.
Mind you, I don't own a "DisplayLink". I use an eVGA UV Plus on my mini. Like many other external monitor devices, however, my eVGA uses the DisplayLink chipset and drivers. So even if you think this solution doesn't apply to your USB monitor solution, it very well may.
I didn't just upgrade to 10.5.7 yesterday. It's been running on my system for ages. So why did everything fail after my log-out/log-in? I seriously have no idea. Following the suggestions on the DisplayLink site and on several Mac support boards, I re-installed the 1.1 drivers and rebooted. My monitor is now back to life.
So, bottom line, if you are using an external USB-powered display on your Mac and it stops working, try re-installing drivers. You'll find the latest Mac drivers here, plus the newest beta releases. You'll need to agree to the DisplayLink EULA before you're permitted to download the software. And you will need to reboot. However, on an annoyance scale between 0 (everything working) and 10 (losing access to your second monitor), a simple reinstall & reboot clocks in at about a 3.