I am developing a decentralized cloud web application, when the user will login into the application he/she will generate password from blockchain, I am not getting how to connect my html form that is username password to blockchain so that it will take the username as the key of metamask and password should be generated from blockchain.
I am sorry, but you are far from understanding how this works.
Ethereum will not generate a user password associated to the Metamask address.
You can let the user to interact with contracts and other entities onchain by letting him to sign messages with their private keys, sign transactions with their private keys.
There's no way to make a login form without using a third service, like a standard oauth system where you store the user password associated with the metamask address of the user, that you can get with
account = web3.eth.defaultAccount;
with web3.js in your html page.
However, unless you are know what you are doing, it doesn't make sense as to interact with Ethereum your user does not need a password but the Metamask passphrase.
You could connect an oauth system to let the user to have access to offchain data, or to authenticate for a third party service, but not to access to Ethereum.