If you forgot to change your patch with a new one that means that you’ve missed the first active pill in the new patch. In such cases, you may take 2 pills the next day (on Tuesday). If you had taken 2 pills on Tuesday you would have been protected and nothing would have been changed in the contraceptive status. Because you didn’t take the first active pill Monday (or 2 on Tuesday) you are not protected and now you should use some back-up protection (condom). You can start another patch when you get your period no matter how many pills left in the present patch.