Few days back, I was not able to connect to internet, when I checked out I found following.

1. There was no Network Connection in 'My Network Places' as well as in Network adapters of 'Device Manager'.

2. The PC light (indicating connectivity to NIC) on my Scientif Atlanta 2100 modem was also off.

3. I am able to access net using USB connection of same modem.

I suspect that my onboard LAN is dead due to lightning. How I can confirm that?

If so, my mobo is under warranty period, so should I go for replacement or simply by a new NIC?

Please Suggest.

The simplest and cheapest thing to try first would be a different network cable. If that makes no difference then get the mobo replaced under warranty as you yourself suggested.

Tried different cable as well, I think NIC is dead, will get mobo replaced.