It's an unavoidable problem due to the demand. A lot of hotels have switched to tiered rates. The rates are lowest when the hotel is empty. As long as you are not at an extended-stay property (where the rate is determined by the length of the stay), you should be able to recover your original rate on the nights you already had booked. Keep in mind that you will need to speak with the hotel or Customer Care, as the reservations line cannot force through such things. If reservations has completed the transaction, prior to discovering the rate problem, have them put you in touch with a Customer Care rep immediately. Marriott Customer care is pretty good about getting rates honored when this happens.