Please help!..there is something wrong with my rabbit!?

This morning when I went to feed my rabbit, he was lying down so I thought nothing of it but even when I tickeld him he just lay there with his eyes half shut. So I went back into the house and came out about 1 hour later but he was still lying there I started to get really worried because this is not like him, he is usually so lively and energetic. I took him out of his hutch and put him in my other rabbit's hutch so he could smell his home while he was in the run but he again just lay down. so I put him on top of his hutch and I was going to check him over for any obvious wounds( I found none) but when I was about to put him in he jumped off the hutch and escaped behind some bins and there is no way I can get to him. He is still there now and has been for about an hour and a half. I'm really worried about him. Has anything like this happened to anyone else, if so what was it, why did your bunny keep lying down and not walking much at all? thank you so muchx