Ameritrade: You have to call them, and promis you wont break the day trading rules again, or deposite 25K.
They will reset your account one time, but after that, if you break the pattern day trader rules, you have to have 25K in your account.
The day trader rules states you are considered a pattern day trader if you make 3 or more round trips within 5 trading days I think. A round trip is a buy and sell of a stock in the same day.
If you buy one day and sell the next, that is not a day trade.