I really need your help! I had u-boot installed with the kernel-power-image and everything worked fine. I uninstalled both, because i didn't need them any longer and my N900 isn't booting anymore. The first boot after the removement it still shows u-boot with opened keyboard but i wasn't able to boot. I flahed the kernel with the original Fiasco-image but it didn't help. Now it doesn't show u-boot anymore. It shows the White Nokia screen then black screen with display light on and after some time it shuts down. Is there something i can do? The problem is, i got very important files on my N900(pics from my son etc) and if i flash it, everything would be gone. It would be so nice of you, if you could help me!

Bumping...
There is a rescue OS, but I have never used it, and I haven't used my N900 for a long time.
Best wishes.

Next time keep backup menu installed and you can flash just rootfs it wont hurt your emmc data . Thats all i know

@Wikiwide: Thank you very much, i read the documentation and it sounds like what i was looking for. I will test it the next days!
@nokiabot: I know about backupmenu, but i don't have the best experience with it. The last time i had an unbootable device after using it. But thanks for your help anyway!

humm i dont use the backup feature but it has moar i.e in this situation you could have used usb networking to transfer the files

If everything failed then flash your device. But flash only rootfs image not emmc. With this method you can boot your N900 without losing your files in internal storage. And also consider proceeding with wikiwide's method.

I finally brought my N900 back to life after having not used it for exactly one year. Reason was, I think, I had the same problem as you. Actually, I was in the rush to install multi-boot so I did a picked the multiboot app from the app manager thinking I'd be able to perform multiboot without needing any other files. Then I was left with a boot looping over and over. I couldn't get it back until tonight when I finally decided to try reflashing it once again. This time, I made sure I was on Windows XP (32-bit), battery charged (just good enough), and all the files I need (flasher, firmware).
I flashed the FIASCO way a couple times. The first time didn't work when I used the global image, but then I flashed again using the USA image, and somehow it magically worked! I was a little bit skeptic while waiting for the screen the loading upon bootup, but after about 5 long minutes, I finally see the Nokia hands animation, and then I threw myself in excitement! I can't believe it; I'm just so damn happy right now. \/
Maybe you should try the other firmware images?

So with rescueOS i was able to use mass-storage but only for /home and not for /home/user/MyDocs (maybe because i got Windows 7). Today i will try just to flash rootfs. Hope this will bring my N900 back to life and keeping my files. I will let you know about the result.

When you brought your device back to life then never ever install kp52 and multiboot together ad this will put your device into bootloop and you have to flash again. And always create a backupmenu backup as its very handy.

Flashing just roofs worked, my files are still there and my N900 is working again. Thank you very very much for your help!!!
So multiboot and u-boot aren't able to work with power kernel?