+1 this thread.
Like MizSuz, had somehow hit the light button and was perplexed how to get it back on until finding the answer here. Have owned the Kobo Aura for a year, but yeah, easy mistake. Signed up to say thanks for the answer peeps!
In case it helps anyone find the answer to this problem faster, I'll fully spell it out for Googlebot ... This thread is a how to fix Kobo Aura light icon disappearing, not working or turning on. In this particular circumstance it's not a firmware issue.