Why does my back and thighs hurt?I barely can walk...?

It may be that small car parked on you.

How the heck are we supposed to know?

This isn't a chat room.

No one on YA can answer this, because it's not enough information and there's no ability to examine you and we are not doctors.
Pain comes from an injury or else a medical condition or illness (such as fibromyalgia, which is just an example--I am not saying you have it), Occasionally pain is just a "mistake" of the brain, but that doesn't seem to apply here. You must have pulled a muscle or strained your back--this often happens while we are active but we don;t get the pain till later so we didn't realize it happened.
First thing is, rest. Lie down. Take it easy for a while. Even if you have to get up, do no sports or active movements (or sex) for a few days. Take an analgesic (ibuprofen, etc.). Get someone to massage your back if you can. A warm/hot bath could be soothing. If it doesn't improve in a couple of days with rest, then go to doctor or chiropractor. Often a chiropractor has more to offer, because unless the doctor knows exactly what caused it (and they may not), they will probably say similar things to what I said.