Not to worry. If the class has stared and you accidently drop the class, As long as you have been attending and participating in the class, you can ask the instructor to send a reinstatement request to the Admissions and Records Office and we will be more than happy to put you back in the class!
If the class has not started and you accidently dropped the class, you can simply reenroll into the class via MyMPC. Note that if there is a waitlist for the class, the next person on the waitlist will be notified to be able to enroll in the class and you will be added to the back of the waitlist.