I also have the problem.
I don't know what could be the reason. I haven't installed any risky applications lately.
I have:
Faster Application Manager
Case File Manager (haven't used it at all, just looked at it)
PeQersi (haven't opened this day)
modRana (it couldn't be the reason, I had it for many days)
Location Test (installed long ago)
Fennec 2 (I don't think I started it today)
Maenotify

So, what occured?
I had about 75% of battery. I plugged in USB cable from computer. It blinked yellow, as usual. Then, after a while, I wanted to do something with it (turn on Mass Storage mode). But the screen was and is black. I can plug N900 in and out, I can press power key or open/close camera or slide the slider, it doesn't react.
It seems at first that it just lost connection to screen. But then, long pressing of power key would make it to turn off or on, and the LED would blink white, and it doesn't.
I'm going to charge it fully until the LED is green, and then, maybe, try to flash it. But I doubt that it will be able to get to flash mode. I cannot even guess whether it's turned on or off.

I don't want to lose MyDocs! I don't want to lose my photographs, sketches, notes!

And I don't want to lose contents of /opt/modrana/tracklogs!

What should I do?

I have already flashed once previously, so I have flasher and 1.2 Maemo 5 image. And it didn't remove contents of MyDocs in that old case, if I remember right.

But what about /opt/ directory?

I have never overclocked N900 or used custom kernel for it.
I have even default transitions.ini.
The only unusual thing I did _this day_ was several hours before the problem. I restarted hildon-desktop several times. But it didn't show any problems at all.

I'm going to sleep, and to charge N900 at night. If it works, it will, at least, show green light in the morning (no battery can charge whole night). And then I'll see whether its screen is still obstinately black. And check replies. What else can I do?

I'll probably have to flash without battery in N900; will /opt/ directory be lost during flashing? MyDocs will not, under normal conditions....