can i enter new blacknetkeypair command! Will the new mnemonic key be valid for the previous public key, for which I did not initially save the mnemonic key

Creating a new mnemonic will make a new key. it won't be linked to a previous key pair.
If you didn't save the mnemonic for the public key you used when burning blk, I think the blackcoin and the blacknet tokens are both lost to the void forever.