How to decrypt passwords that are encrypted in an SQL database [C#]

Question

so I am having a problem with this script I have. I created a new class in my project and it is named AesCryp.cs and it comes with both encryption and decryption methods. Here is the script:

class AesCryp
{
    public static string IV = "xxxxxxxxxxxxxxxx";
    public static string Key = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";

    public static string Encrypt(string decrypted)
    {
        byte[] textbytes = ASCIIEncoding.ASCII.GetBytes(decrypted);
        AesCryptoServiceProvider encdec = new AesCryptoServiceProvider();
        encdec.BlockSize = 128;
        encdec.KeySize = 256;
        encdec.Key = ASCIIEncoding.ASCII.GetBytes(Key);
        encdec.IV = ASCIIEncoding.ASCII.GetBytes(IV);
        encdec.Padding = PaddingMode.PKCS7;
        encdec.Mode = CipherMode.CBC;

        ICryptoTransform icrypt = encdec.CreateEncryptor(encdec.Key, encdec.IV);

        byte[] enc = icrypt.TransformFinalBlock(textbytes, 0, textbytes.Length);
        icrypt.Dispose();

        return Convert.ToBase64String(enc);
    }

    public static string Decrypt(string encrypted)
    {
        byte[] encbytes = Convert.FromBase64String(encrypted);
        AesCryptoServiceProvider encdec = new AesCryptoServiceProvider();
        encdec.BlockSize = 128;
        encdec.KeySize = 256;
        encdec.Key = ASCIIEncoding.ASCII.GetBytes(Key);
        encdec.IV = ASCIIEncoding.ASCII.GetBytes(IV);
        encdec.Padding = PaddingMode.PKCS7;
        encdec.Mode = CipherMode.CBC;

        ICryptoTransform icrypt = encdec.CreateDecryptor(encdec.Key, encdec.IV);

        byte[] dec = icrypt.TransformFinalBlock(encbytes, 0, encbytes.Length);
        icrypt.Dispose();

        return ASCIIEncoding.ASCII.GetString(dec);
    }
}

I have made it when I create a new account, it will encrypt the password and insert it into the database. Here is the register script:

public bool Register(string user, string pass)
    {
        string query = $"INSERT INTO users (ID, Username, Password) VALUES ('', '{user}', '{pass}');";

        try
        {
            if (OpenConnection())
            {
                MySqlCommand cmd = new MySqlCommand(query, db);

                try
                {
                    cmd.ExecuteNonQuery();
                    return true;
                }
                catch (Exception ex)
                {
                    return false;
                }
            }
            else
            {
                db.Close();
                return false;
            }
        }
        catch (Exception ex)
        {
            db.Close();
            return false;
        }
    }

and here is the SignUp button script:

private void SignUp_Click(object sender, EventArgs e)
    {
        string user = usernameTxt.Text;
        string pass = AesCryp.Encrypt(passwordTxt.Text);

        if(Register(user, pass))
        {
            MessageBox.Show($"User {user} has been created!");
        }
        else
        {
            MessageBox.Show($"User {user} has not been created!");
        }

    }

However, I am just really confused on how I am going to make it so it decrypts the encrypted password in the database when I try to login. This is my login script without any decryption (because I'm not entirely sure how I would do it)

public bool IsLogin(string user, string pass)
    {
        string query = $"SELECT * FROM users WHERE Username='{user}' AND Password='{pass}';";

        try
        {
            if (OpenConnection())
            {
                MySqlCommand cmd = new MySqlCommand(query, conn);
                MySqlDataReader reader = cmd.ExecuteReader();

                if (reader.Read())
                {
                    reader.Close();
                    conn.Close();
                    return true;
                }
                else
                {
                    reader.Close();
                    conn.Close();
                    return false;
                }
            }
            else
            {
                conn.Close();
                return false;
            }
        }
        catch (Exception ex)
        {
            conn.Close();
            return false;
        }
    }

and here is the Login button script:

private void Login_Click(object sender, EventArgs e)
    {
        string user = usernameTxt.Text;
        string pass = passwordTxt.Text;

        if (IsLogin(user, pass))
        {
            MessageBox.Show($"Welcome {user}!");
        }
        else
        {
            MessageBox.Show("Username or password is incorrect!");
        }
    }

If anyone could help me out, then that would be great. Thank you!