title_body: Difference between Brew and Keep Warm?
upvoted_answer: Normally in a drip coffee machine the 'brew' mode is where the water in the reservoir is boiled/heated and dripped over the ground coffee, before passing through the filter and into the jug. The 'keep warm' part is usually a hot plate under the jug which heats the jug from below, keeping the coffee warm.
  
  My drip coffee machine doesn't allow you to switch it to one mode or the other, you just switch it on and it pumps the water through the filter until its empty - there is a floating switch in the bottom so it knows when there is no more water left, and the hot plate is always on.
  
  I assume that your drip coffee machine works in the same way but has separate controls for the brew and keep warm features. Switching the 'keep warm' feature back on isn't going to harm anything - I'd question the quality of the coffee after 2+ hours sitting on a hot plate though.