Why is my external Mac keyboard not operating?

Hi,

I bought a MacBook Air (13-inch, Early 2015), and I also bought a Bluetooth keyboard for it which doesn't seem to use a conventional battery.

It was working in June but when I came back from vacation in September it wouldn't interface with my MBook Air even though both are listed in the Bluetooth window and Bluetooth itself is enabled.

Any ideas? Thanks a lot,

MF