My friend got a special problem he said that his pc is not starting when it is settled normally on the table but wen he set the pc on the ground it works normally what should be the reason ??? please help him !!!

Sounds like a bad power cord/cable.

If it is a laptop it could also be an internal broken power connector.
Take out the battery, try wiggling the power plug, watch the AC power indicator light see if it's flashes while wiggling the plug.

Where the adapter plugs into the laptop can take a lot of strain. This is a common problem with certain model notebooks. I worked at a repair depot and on one specific model of Sony, 99% of the repairs for that model were new power plugs on the motherboard.

sir its not a laptop its pc i am talking about pc wen i put it down it worked for once now its totally stopped responding wen i switch it on some sort of sound is coming so wat i have to do