The ‘admin’ password is stored in MONyog.ini (passwords for other MONyog users – if you have such – are stored elsewhere).
Just open MONyog.ini in an editor and blank the password so that the relevant line looks like
.. now restart MONyog and you can login as ‘admin’ with an empty password. After that set a new password from the MONyog interface .. tools .. change password. The new password has effect after next MONyog restart.