Maybe you just got too involved with a project, and forgot to eat. But you kept nursing the caffeine-drink right along... I've done this too many times!
Here's a simple and quick fix. Eat a banana.
The good news is that you can usually find one, quickly, at a nearby convenience or even drug store. More good news: a bunch of five or so will keep pretty well, in your desk, if you don't mind a bruise or two, on them.
The blog for Livestrong tells Lifehacker that you might also try a little milk or cheese. The potassium, calcium and protein help get us back in good order, and back in action. :)