Urgent support with wireless keyboards for iMac

I Really need help with my wireless keyboard problem connecting to my iMac Late 2013.

i left my iMac on sleep mode whilst going out but turned off the wireless apple keyboard and regular mouse and came back a few hours later. I was able to move my mouse fine whilst on the enter password screen however I realised my keyboard did not work. with several attempts to turn the device on and off again. The light would turn to green then to black and begin to blink for a while meaning it is ready to pair. However, I can't do anything to pair, it won't automatically pair and also I am unable to enter system preferences to troubleshoot this. So I really need immediate assistance Asap. Thank you so much.

The only things i can do is just turn on the iMac with the back power button and use my mouse to click on cancel option in the bottom Middle Of the monitor or in the box to place my text for the passcode/word.

There is also a skype message in the top-right corner of my screen frozen.