My office workstation machine has lately been "losing" the graphics card EVERY time I reboot the machine.
It's a Dell Precision 490 with dual dual-core Xeon 2.67 GHz CPU's, 4 GB DDR2 memory, and an nVIDIA Quadro FX550 128MB graphics card. The machine is nearly 3 years old. I have two 19" 1280x1024 LCD panels hooked up via DVI to the graphics card, with the Windows desktop stretched across both displays to 2560x1024.
In recent weeks, every time I reboot the machine, the desktop comes up at 1280x1024, cloned on each display. When I look in Hardware Manager, there's a yellow exclamation mark next to the graphics card, and it says it could not start or some such thing. There is also no nVIDIA Options choice when I right-click on the desktop.
So every time this happens, I end up re-installing the nVIDIA driver (latest version), restarting, and then it seems fine. But then the next time I reboot, I'll have the same problem again.
It sounds suspiciously like what happens when you have two devices whose drivers conflict with one another. With video cards, it's usually a built-in graphics adapter coming on by default and preventing the system from recognizing the video card in the slot. But since your machine was working fine, that doesn't make sense unless you messed with the BIOS recently and re-enabled the onboard video.
Since you have a fairly veteran machine, one possible explanation COULD be that your CMOS battery went dead, so every time you let the system power down completely, the BIOS gets wiped clean and defaults to original factory settings -- i.e. onboard graphics switched on. The CMOS battery usually lasts for a long time and some newer systems have safeguards against a dead battery as well, but you could have gotten unlucky. A dead giveaway would be if the system date and time keeps spontaneously resetting itself.
Other than that, did you install any other new devices recently -- even anything as simple as a camera, a printer, anything like that? It's pretty rare, but sometimes a new device's driver will conflict with a completely different device's driver that's already installed. Not long ago, I installed a webcam, and all of a sudden my machine couldn't recognize the printer that had been hooked up for two years. Never heard of it happening with a video card, but I suppose anything's possible. If you're getting a message like "This device cannot find enough free resources that it can use, you will need to disable one of the other devices on this system," then maybe that's worth looking into.
Don't know if either of these will solve your problem, but it's the best guess I've got.
I haven't installed anything on it recently that I can think of.
What's weird is that I rebooted several times this morning in preparation to install a new HDD and Win7, and I never had a problem at all!
So I still have the FX550 card installed, but now with a new HDD and Win7 x64, and so far it seems to be working fine. Win7 already had a fairly recent nVIDIA driver for the card to use, so I didn't even have to install anything graphics card-related when I installed the OS. Hopefully it will stay that way.