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
Password=
.. 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.