ifconfig wlan0 up

-OR-

sudo ifconfig wlan0 up

Edit: I do not know why you would disable wifi, if you disable wifi it would remove wlan modules and you won't be able to get an interface.

Hi tuxsavvy,

I tried it using root login,

the output I got was,

ifconfig: SIOCSIFFLAGS: No such device

You probably do not have the modules loaded. Make sure that you do not have wifi disabled with wifi switcher.

Also check inside `lsmod' for wl12xx/wl1251.

the reason I disabled wlan was thinking that it would extend the battery life of n900 using wifi-switcher. but that thing totally screwed my wlan.

now I regret using it. If you want I can start afresh by reflashing eMMC + firmware.

It doesn't exactly screw up with your wlan, it just removes the kernel modules (or simply put drivers) for wireless lan. When you enable it again it will load the kernel modules.

Although there's lots of rumours saying that it can save battery life but I'm guessing if you disabled automatically connect (under Settings > Internet Connections), the amount of power saved with that disabled is literally neglegible.

So you can re-enable wlan by pressing on the wifi switcher.

I have removed wifi-switcher. restarted the phone and logged in as root.

tried the command 'lsmod' and I could see wl12xx inside.

now I tried the command

'ifconfig wlan0 up'

I still get the same message.

Paste me your dmesg output (or pastebin if you wish).

I have attached the dmesg out as a text file.

Found the lines causing the issue:
That could either be a serious error or something else is causing the wireless LAN to not work. I'd try reinstalling wlan switcher and turning it on and off a few times, trying to see if you can get wireless connectivity in between on and off.

If that fails then I presume reflash would be in order. I have never seen that error message before.

Thanks a lot for your help. I will try that and get back to you. I appreciate your help a lot mate.