My N900 is getting restart automaticaly and became unusable....as its power on then again it get restart..i m very worried of it....

pls anyone help me pls...

Some more info:
when i go this

$ cat /proc/bootreason
sw_rst

i have flashed with latest firmware also same problem.

please guy help me out..

Have you tried leaving the device charging overnight, then unplug the charger, wait 30 seconds and try to switch the phone on?

Someone else in another thread fixed this by removing the battery, SIM card, MicroSD card (if you have one) and then waiting 30 seconds and carefully putting them all back in. Do your best to charge the battery first.

Otherwise google:

maemo wiki updating the firmware

and follow the instructions there.