f3 expose not working

I have macbook pro got it last year and the f3 button which exposes the windows i have open is not working. When i press the button basicly nothing happens no dimming of light no nothing. I remember it happened before a month or so ago and i fixed it by following someone elses solution but now i cant find where i got that information from. I think i remember going into terminal typing something and then after going to macintosh, library then preferences but i cant remeember what to do.

If any body knows a solution that would be great. I have 3 windows open so its not a chase of having just 1 window open it was working fine last night. also does anyone know what triggers it, my mac completely ran out of battery would that be why?

Many thanks