It happens. If I were a blonde country singer, I might say "Shh...it happens." You're rolling along, wind in your sails (and voila, a tired cliche already), and it happens. Something unexpected. Something unwelcome.
I could go a bit cliche crazy - lemons and lemonade, God closes a door, opens a window. I could quote Nietzsche. I won't.
Shit happens. You have my permission to moan about it for a day or two (and in case you hadn't noticed, this is my day), then you get on with things. Like 1,250 words a day.