Try with someone else's SIM card. That way you will know if it's your SIM or the phone.

the strangest thing is that when the phone is connected to the pc in mass storage mode
the network is available, when disconnected it is not available again

Have you tried reflashing? Do a backup with the backup app to either an SD card or your PC, then follow the instructions here:
http://wiki.maemo.org/Updating_the_firmware
Then using thne backup app restore your data.

it is difficult coz i'm not a computer expert but i will try

Try the sim card in another phone. I quite often get that message... when I'm in an area where the cellular network is not available, oddly.

It is faintly possible that the network is having a bad day.

Second, if it persists, reboot. Yes, know that sounds very basic, but if you have passed through a no network area (or moment) and the device can't reconnect, a reboot will often enable it. This has happened to me on n95 as well as n900 (though only on a particular network).

1-i tried the SIM in another cell , it works
2-when i reboot, the network is available for a while then it isn't again

i just flashed my n900 with global release
RX-51_2009SE_1.2009.42-11_PR_COMBINED_MR0_ARM.bin
RX-51_2009SE_1.2009.41-1.VANILLA_PR_EMMC_MR0_ARM.bin

and now it doesn't see my sim card

found this thread through google.
help pls

im bloody idiot. nwm.
flashed an old firmware i think. gona reflash with the new one.

upd
yes its all working now. used
RX-51_2009SE_20.2010.36-2_PR_COMBINED_MR0_ARM.bin
RX-51_2009SE_10.2010.13-2.VANILLA_PR_EMMC_MR0_ARM.bin

Dear Sir

My Mobile is Nokia N900

I was have the same problem, and I search for many hours to solve my problem, then finaly I solve it by flashing the following image:

RX-51_2009SE_20.2010.36-2.003_PR_COMBINED_003_ARM.bin

I found it in the following URL:

http://tablets-dev.nokia.com/nokia_N900.php

It is for middle East and north Africa.

Or may be the combined image of your country will solve the problem.

At all, after I flashed this image my cellular Network work correctly, and the mobile recognize the Sim.

I flash it by flasher 3.5, which can be found in the following URL:

http://tablets-dev.nokia.com/maemo-d...-downloads.php

After installing of flasher ((Not in 64-Bit operating system)) , put the image file in the:
%systemdrive%\Program Files\maemo\flasher-3.5

Then Copy the following command:

flasher-3.5.exe -F RX-51_2009SE_20.2010.36-2.003_PR_COMBINED_003_ARM.bin -f -R

Not that the text in red color is the name of image file.

Then past it in the CMD window by clicking right hand then clicking past.

after that follow flashing process as shown in the following URL:

https://wiki.maemo.org/Documentation..._flashing_mode


Best Regards
AL-Mohamadi
Yemen Arabian

Please Replay if you solve your Problem for advantages.