We use "must" to describe something that is unavoidable or required. We use "should" describe something that is suggested.
I should go to work today, but I don't feel like it. (suggested)
I must go to work; my company depends upon me. (required)
She must not feel sick, we need her at work. (We wish that she is not sick.) She cannot be sick for all this time. (It is not possible for her to be sick for that length of time.)