Hello,
There is what is call a belt tensioner assembly, you will need to use a special tool in order to move the tensioner so that the belt can be reinstalled. However, there may be a problem if the belt just came off and you should have it checked.
Take a look at the link below and find the set up for your vehicle and it will give you the location of the tensioner assembly.
https://www.2carpros.com/car_repair_information/year/2004/make/mercury/model/sable/2004_mercury_sable_drive_belt_routing_diagram.htm
Here is the special tool that will make it easier to get the belt back on....

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Mar 29, 2009 at 2:33 PM