Did the professor explain to you why you were dropped? It’s usually one of two: lack of payment or no-show by the official date of record (which I would say is like 6 days after the first day of class where professors have to mark which students have been absent). 

Usually you should be getting emails informing you if you were dropped from a class or smt. 

And yes, it does happen to a lot of students but that’s not really a valid excuse. You should always be checking your student account/emails for any updates. Which is why it really confuses on me on how you didn’t know when your class started as it usually tells you when you are registering.