Java Login Codes Reading multiple lines from file not working

Let's analyse this section:

while (scanner.hasNextLine()) {
    String fileline = scanner.nextLine();
    if (!(fileline.equals(hashedP))) {
        System.out.println("Login Failed!");
        LoginMenu();
    }
    else {
        System.out.println("Login Successful.\n");
        AdminMenu();
    }
}

1. We enter the while loop.
2. We read the first line from the file.
3. We check it against hashedP.
   3.1. If it matches, we display the admin screen.
   3.2. If it does not match, we prompt the user to log in again.

You never even reach the second line in the file, it fails too fast.
You should refactor your loop to try harder:

boolean failed = true;
while (scanner.hasNextLine())
{
    String fileline = scanner.nextLine();
    if (fileline.equals(hashedP))
    {
        failed = false;
        System.out.println("Login Successful.\n");
        AdminMenu();
    }
}
if(failed)
{
    System.out.println("Login Failed!");
    LoginMenu();
}

Unrelated to that, it's never a good idea to call a function recursively if in any way avoidable.
In this case, for example, admin.dat and a new stdin Scanner will be opened as many times as an incorrect password is entered, which is poorly designed.
I suggest using a while(true) loop instead to read the password and doing everything else before that:

public static void LoginMenu()
{
    ArrayList<String> hashes = new ArrayList<String>();
    File file = new File("admin.dat");
    try
    {
        Scanner scanner = new Scanner(file);
        while (scanner.hasNextLine())
        {
            hashes.add(scanner.nextLine());
        }
        scanner.close();
    }
    catch (FileNotFoundException exc)
    {
        exc.printStackTrace();
        return;
    }
    Scanner UserPass = new Scanner(System.in);
    while (true)
    {
        System.out.println("Please Enter Your Password: ");
        String UserP = UserPass.nextLine();
        String hashedP = Utility.getHash(UserP);
        if (hashes.contains(hashedP))
        {
            System.out.println("Login Successful.\n");
            AdminMenu();
            break;
        }
        System.out.println("Login Failed!");
    }
}