Where do exactly bitcoins exist? If Someone has bitcoin in a Wallet, then suppose that the company of the wallet get closed or finished work or destroyed, how could one restore his bitcoin? 

Where exactly do the bitcoins exist? Are they "exist" in the blockchain and can be resored from there?
Bitcoins are stored in the blockchain in the form of unspent transaction outputs, i.e. the product of confirmed transactions. These unspent transaction outputs can be spent if you control the private key corresponding to the address they were signed over to. If a private key is lost irretrievably, the coins will never become spendable again. There is no key recovery service. You're responsible of creating sufficient backups and keeping your private keys safe.

If a company shuts down their service, they should pay out any remaining balances to their users. If they have full custody and go out of business without returning balances, you're probably (depending on the exact circumstances) out ouf luck.