I am fuming, due an important call later today and my N900 has decided to go haywire!

I have made sure its not in offline mode, I have rebooted, I have taken the battery out for a few minutes, taken the sim out, everything I can think of, but the phone doesnt do anything!

It switches on ok and all the screens are there, but there is no wi-fi signal for internet access, unable to make calls (or receive them) and unable to send (or receive) sms.

In all intents and purposes - it is dead!

How the hell do I fix this? I have not made any updates to the software and it was fine yesterday!

Please do not double post

http://talk.maemo.org/showthread.php?t=68980

As for your issue, I would suggest trying a new SIM card

Sorry did not mean to double post - only as the Cellular data message did not appear before and I thought it was just an internet problem, only to find out later it's everything.

Sim Card is fine - tried it in a friends phone.

Would a roll back to an earlier software version work?

No, older software has a different gsm module hence will not work. Once upgraded to the latest version, you can't go back (least not easily). What exactly happened? Did it suddenly stop working? Did you flash it and then it stopped?

Did absolutely nothing - that is the strange thing!

Just noticed this morning that there was no wi-fi signal ( was working fine yesterday - received calls, made texts, browsed the net )

Then realised could not make a call or send a txt ( cellular data error appears )

Tried everythign I can think of - not downloaded any applications, reflashed, nothing

You need to flash the phone then, using the latest firmware.

http://wiki.maemo.org/Updating_the_firmware

If after a complete reflash it still doesn't work, then you probably have a hardware problem.