I have just followed advice at http://www.nutstech.info/2013/05/how-to-... and have followed this procedure:
Turn off the keyboard.
While holding onto CAPS lock, keep pressing a few keys for the next 5 or more seconds.
Turn on the keyboard.
It worked for me.
I may also have a weak battery (the red LED flashes when I press the test putton), but at least I can type with it again. My keyboard is about 4 years old and this is the first time it has stopped working.