Why do my bike gears seem so different all of a sudden?

Today I got on my bike, on the highest gear like I always have it, and it seems like it is much lower. I need to pedal so fast to get any resistance. The chain looks like it's on as always, and it shifts, but the lower gears are so ridiculous now I can't even move my legs that fast. How could thi have happened? How do I fix it?