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First look on top of the fan and see if you find a set of dip switches. If not, you may have to lower the cone shaped canopy to find the receiver. The dip switches must match what is set in the remote.
If the receiver is in the canopy, it will be similar to this:
Match the dip switches either ON or OFF with the receiver setting and the remote setting. The remote switches are usually next to or behind the battery
It happens many times when changing a battery, to accidentally move one or more switches on the setting.