Why is she refusing to nap?

My daughter hasn't had a nap in over a week! She's driving me craaaazzy! She sleeps from about 8:30 at night to about 8:30 in the morning. She used to take a 2 hour nap at 11:30 and an hour nap at about 4, but this week she has refused to take a nap at all. I can get her to take a cat nap for about 10 minutes if I'm holding her, but as soon as I put her down she wakes up. I've tried naps in the swing, bouncer, and crib, but nothing. I've also taken her for long walks and car rides. She gets so sleepy and cranky. Anyone else's baby do this? What could be causing the sudden change? And how do you get your little ones to nap?