What happens to Dash wallet if PC is formatted?
I haven't used Dash, but they say it's based on Bitcoin-QT (from Bitcoin Core).  Assuming they operate similarly, then in order to recover coins, you need both your wallet.dat file and your passphrase.  If you don't have them both, your coins would be lost.  So you should keep secure and reliable backups.

Some wallet clients can give you a "mnemonic phrase" (not the same as a passphrase) which can be used all by itself to recover your coins.  However, Bitcoin Core is not one of them, so I assume Dash doesn't support this either.