Today we experienced a Network disconnection soon after a thunder and lightening. We have switched off and on all the equipment that part of our network, i.e. modem, router, switch and server but still not help. What happened now is that our server is down and we do not have any idea what is the real problem. Please help us solve this problem.

Could be damage from a power surge.

I second that. The only way you'll solve this is with trial and error and removing certain parts, testing them with others etc.