I flashed my N900 with some latest OS from maemo.org. It was good. Then I have install some soft for camera and I lost all sources (503 service unavailable via wifi or umts). I reflashed with default firmware and problem still exists. I cannot find here any similar thread with solution.
How to step by step clean all firmware (I have read that sometimes re flashing with first versions of firmware than updating is more sufficient for battery life).

I'm completely green.

I guess you missed that Nokia shut down everything.
Install CSSU
http://talk.maemo.org/showthread.php?t=80392

I tried to install CSSU. I have added sources from maemo.org. And still I have 503 error.

But I cannot do anything via terminal (with root) - I've got "Could not open lock file /var/lib/dpkg/lock - open (13 Permission denied)
Unable to lock the administration directory (/var/lib/dpkg/), are you root"

Shall I do smth with Linux and dedicated tool to reflash with CSSU?

Hello.

Please see http://talk.maemo.org/showpost.php?p...&postcount=112 - read the rest of that thread to gain more knowledge, but basically you need to download 2 files attached to that post, remove ".txt" extention form them and open them on N900 - it will add unofficial backups of Nokia repos that were shut down. It's hard to install something on "fresh" N900 these days without them. There are many threads about that, but maybe you searched without proper keywords

Next, see https://wiki.maemo.org/N900_The_Perfect_Setup , especially parts about CSSU, power kernel and swap on sd.

If you get this message in terminal ("are you root?") it states you need to be root to perform some operations. Please open Hildon Application Manager (the gui app manager from N900's menu) and install the package that is called "rootsh" as far as I remember - then you will be able to type "sudo gainroot" in terminal to increase your permission level and perform some operations.

Good luck and have fun, but I feel like you're not experienced with linux, so please know you might break stuff (I suggest you install backupmenu and make a full backup once you got cssu and kernel power installed and everything configured).

@Misiak
Thanks I will try. (I was using a linux at PC, so i had to do with terminal and some commends). I am not sure about new sources, but I will try.
Break? Definitely? I dont care for any data. Just want to bring back power over OS on my N900 (need to install email, web browser, portrait switch, vlc, pdf reader).
May I ask u to contact with You via email? If U agree please send me a meesage (just add to my nickname @gmail.com)
I will follow Your instructions.

alternative repos are also giving errors now. any fix for it ?

http://talk.maemo.org/showthread.php?t=94722