Talking in the present about something that is almost certainly the case, something that is highly probable.

POSITIVE statements: Use "must" + <infinitive without 'to'>.
It is almost certain that you are very tired because you have been working all day.
You must be tired because you have been working all day.

POSITIVE statements: Use "must" + <continuous form>.
It is almost certain that you think that I am drunk because I've had a lot to drink.
You must be thinking that I am drunk because I've had a lot to drink

NEGATIVE statements. Use "can't" + <infinitive without 'to'>.
It is almost certain that it is not working because it is switched off.
It can't be working because it is switched off.

The negative of  must  in this case is can't.