If I know the sha256 hash of a password, can I use IPFS to reverse it and get the plaintext password?
The short answer is: No.
The slightly longer answer is that it would require adding every possible password with every possible salt value as files to ipfs. For sha256, that works be 2^256 (or about 115.7 x 10^75) entries for a single salt value, which is more than there are atoms that make up the planet earth.
If you’ve lost your password, IPFS won’t let you recover it.