When did this started to happen? Were you making any changes to the Wi-Fi settings or network settings? Or did it just out of the blue started to fail?
If you were any changes to the network settings, it would be great to know what you were trying to achieve. But in case the Edison just stopped connecting to the Wi-Fi access point, then I'd suggest you to reflash your board, this issue is not very common and it can probably be resolved by flashing the latest image again.