BTW, this has personally happened to me twice in the past month. Here is an example of the email:
Hello,
We're sorry, your previously confirmed FastPass+ selection for Big Thunder Mountain Railroad® from 5:30 PM to 6:30 PM on 03/02/2014 needs to be changed because the experience is temporarily unavailable.
As a replacement, you may now visit any one of the following FastPass+ experiences on 03/02/2014, starting at 5:30 PM through anytime during normal Theme Park operating hours (Valid Theme Park admission is required and your FastPass+ selection cannot be redeemed during Extra Magic Hours):
Experiences*
Pirates of the Caribbean®
Monsters, Inc. Laugh Floor®
Haunted Mansion®
Tomorrowland Speedway
Big Thunder Mountain Railroad®
Space Mountain®
Buzz Lightyear's Space Ranger Spin®
There is no action needed to select the experience in advance. During the designated day and time, just arrive at the FastPass+ entrance to one of the experiences above and touch your MagicBand to enter!
*Note that you may be able to still experience your selection for Big Thunder Mountain Railroad® if it becomes available again today, unless you opt to visit one of the other listed experiences.
Visit My Plans on My Disney Experience for more details.