Quantum computing isn't like a magic wand that you can simply wave at cryptography to break it. It is just that there are certain mathematical problems that can be solved significantly faster using quantum processing as opposed to current processors. Some of these mathematical problems apply to Bitcoin and are thus potentially vulnerable to quantum computing. Luckily however Bitcoin also contains mathematical problems for which quantum processors don't have an edge over current processors.
Nothing is instant and even quantum computers require the public key before being able to derive the private key eventually. Luckily the public key of an address is not known until the first transaction has been made, ie. your coins should be quantum safe as long as you refrain from reusing addresses.
Even if Sha256 remains the PoW hashing algorithm of choice for Bitcoin, quantum computers would likely simply displace ASICs as mining hardware of choice. Why attack a blockchain if you might as well just mine it?