Here is the response I got after sending an e-mail:

Dear XXXXXXX,

Thank you for contacting Marriott. We appreciate the opportunity to provide you with information. Please accept our apology for the difficulty you encountered while using the Marriott.com "Look up reservations" function. We are eager to ensure that Marriott.com is a productive part of your travel planning process and understand that it is frustrating when it does not work as expected. In order to enhance your Marriott.com experience, we recently updated certain aspects of our website. Unfortunately, this resulted in some members not being able to view their upcoming reservations. We are researching this issue and will have it corrected soon.

If you wish, we are more than happy to send a complete listing of your upcoming reservations. Please respond to this email, and we will provide the information as soon as possible. If you require additional assistance by telephone, please contact an Internet Customer Care Associate by phone at 800-932-2198, from within the United States or Canada. We are available to help you 24 hours per day, 7 days a week. For assistance with your reservation if you are outside the United States or Canada, please consult the following web address to locate the telephone number of a Marriott reservation office in your area: https://marriott.com/reservation/worldnum.mi We look forward to assisting you.

Best Regards,
Travis Norton
Marriott Internet Customer Care