Apple Keyboard not discoverable

I've got a problem.. I'm a user of iMac from late 2011. I had some problems with connesting my keyboard to my mac but they always were solved. This time I can't connect my keyboard to computer no matter what I do. I tried resterting it, holding the power button for couple seconds etc. I found the advise saing that I should unpair the keyboard with my comp. Now the problem is that the keyboard IS in discoverable mode (the light is blining) but the computer can't see it :/ .

I tried everything. Please help me!