It sounds trivial: Use strcmp or isequal to compare both strings. If they are different, repeat the password input dialog.
I assume you have a specific problem. Please post the current code and explain what you want to have changed.
However, password protection is a extremely tricky process. Inside an M- or P-file a cheating is trivial. Never store passwords in clear text, because it is very easy to extract them from the function. Better use a hash, e.g. MD5 or SHA-256. And then add "salt" to the password to make a decryption too expensive.
