Decrypt from SHA256

Solution 1:

You cannot decrypt the result of a One Way Hash. What you should do instead is compare a hash of the entered password versus the stored hash in the database.

Example:

var password = "1234";
var hashedPassword = Sha256encrypt(password);

var allowLogin = hashedPassword == storedPassword; //storedPassword from Database, etc.

This is only the very basics though, when using hashing algorithms you should consider using a Salt too.

Solution 2:

It is impossible per se. SHA is a hash function, which implies it is one-way and used just for validation and similar things. Since the result of SHA-256 is of fixed length (256 bits) that also means that most of the information is lost when computing it.

You can brute-force it though, meaning that you could try and compute hash of a large number of different inputs and see if the hash matches.

Sometime in the future a cryptographic weakness may be found for SHA thus making it breakable but practically it is not a reversible function.

See details about hash functions on Wikipedia.