my phone is not starting after uninstalling linux kernel. i tried to reflash but when i connect usb pressing 'u' it doesnt show any icon on screen of nokia n900 .
when i just connect usb, a yellow LED lights for a minute and goes off and then vibrates, a dull nokia screen appears, this repeats till disconnection of usb.

you should tyr pressing u and then connecting the usb. you should also change usb pc ports.

i tried but still its not working

This happens all the time. You can't just uninstall the kernel like that. There is a special process that you didn't follow.

The easiest way to reflash is from Linux. You can easily run Ubuntu from a cd and follow the instructions.

You can also reflash from the command line in Windows.

There are instructions for how to do these things. You have to tell us exactly what you did and what doesn't work.

The great thing about doing it in Linux is that timing isn't much of an issue. Flashing starts pretty automatically once you have taken the right steps.

Ubuntu Linux is fun to install. I suggest you start with that, on a boot disk.

First make sure it has really shut down: After disconnecting the USB wait some 30 seconds, then pull the battery.
Then only flash the kernel from the fiasco image that correspondents to the firmware of your phone according to the wiki wiki.maemo.org/Updating_the_tablet_firmware . Do it from an Ubuntu- or Debian-based Live-CD.
Install kernel-maemo and kernel-maemo-flasher, nevertheless, later.