Apple has announced that MobileMe will be discontinued on June 30, 2012, and that iDisk will not be available after that date. Apple stated that they will not migrate iDisk service from MobileMe to iCloud.
Did you convert over to iCloud from MobileMe. If so, MobileMe services may not be available to you. While iDisk is available through June its probably constrained to those who use MM and have not migrated to iCloud