Why won't my Apple wireless keyboard turn on?

I have a wireless Apple keyboard (about 7-8 years old)that will not turn on. When I press the power button the green LED lights up for about 3 seconds then goes out. It does not flash as though trying to make a connection to my IPad. During that 3 seconds the Bluetooth window detects the keyboard but pushing the "connect" doesn't work as it powers off too quickly.

I put fresh batteries in and still get the same results. The battery terminals are clean. No corrosion ever.

The last time I used it, it worked perfectly.

Please, any suggestions? Thank you.