It happened to a friend of mine before :

1- Hold the red button for syncing (the one on your Wii) for 10 seconds.
2- Unplug your Wii.
3- Take out the batteries
4- Reconnect your Wii and put the batteries back into the Wiimote.
5- Turn on your Wii and try syncing and it should work.

This came from Nintendo when he called them to solve the problem. If it didn't work, try holding the red button on the wiimote for 10 seconds instead, I can't remember which one you had to hold but I'm 90% sure it was the Wii one.