The default administrator login credentials for a new Web Portal installation is Administrator for both username and password. If this password for the Administrator login was changed and needs to be retrieved, you will have to reset the password in the database back to default. The below query can be used to reset the password back to the default of "Administrator".
UPDATE MIR_META_ATTRIBUTE SET VAL='7b7bc2512ee1fedcd76bdc68926d4f7b' WHERE MODEL_ID=-1 AND CLASS_ID=-4 AND ATTRIBUTE_DEFINITION_ID=265;
After changing the password back to default, you can now login using Administrator/Administrator. You can also change the password if you'd like by going to Tools | Administration | Administrator | change password