Install new batteries in the remote control if the fan is not responding to the remote's commands. This is far and away the most common cause of a remote's no longer controlling a ceiling fan. Remove the batteries now in the remote and install a new set per the manufacturer's directions. Once the new batteries are installed, try the remote again to see if it is now able to control the fan.
Check dip switch settings on the remote and on the fan if installing new batteries did not resolve the issue. Unlike a universal remote for a TV, ceiling fan remotes use tiny dip switches that can be placed in either an up or down position in order to communicate with the fan. Consult the manufacturer's directions on both the remote set and the fan itself and reposition any dip switches as necessary. Once you verify that all the switches are in the proper position, check the fan's controls again.
Test the controls on the fan itself. Having ruled out dead batteries or misplaced dip switches as possible causes of the problem, it is time to check and see if the fan itself is causing the problem. Use the chains and/or switches on the fan itself to adjust the fan's speed and direction as well as to turn the light on and off. If there is no response from the fan's controls, the fan itself is the most likely cause of the problem and you will need to examine the electrical connections to the fan or possibly have the fan serviced. If the fan's controls do respond, the remote is malfunctioning and will need to be replaced.