It seems like all of a sudden several people I know all went into a funk simultaneously. I started noticing I was sliding that direction as well and started my standard remedy. It seems to be working nicely, so I thought I would post it.
My secret fix is a daily does of a good multi-vitamin with an extra supplement of B-6. (The one I take is a 50 mg supplement.) Be careful with this, since it is possible to overdoes on B-6. The extra B-6 is particularly important for women, who tend toward a B-6 deficiency. Stop taking it if you notice that you have very active, not restful dreams. This fix I find will pull me out of just about anything within about three days. (Though don't forget to eat at least three solid meals a day -- no junk food! -- and get reasonable amounts of sleep. Do both of these at "normal" times of the day, whenever that may be for you.)
Another thing to try is apple cider vinegar. The recommendation I saw was to mix a tablespoon of vinegar with 8 ounces of water (mix in a tablespoon of honey if that is too tart for you.) Personally, I like vinegar, so I will just drink the tablespoon down straight (but make sure to get the 8 ounces of water down as well!)