My ferret is sick...?

My ferret Spunky, is around 6 years old. He's gotten VERY VERY VERY skinny. And, when I let him out, he keeps running under a blanket, or somewhere dark, and just laying there. This is NOT like Spunky. He's normally a very playful ferret. Please help me. This was my very first pet, and I'd hate to lose my pet bestfriend.