I've had that feeling. I've been so down I haven't left the house for days! But I get sick of myself and get up, take a shower and find something to do.
I know it's hard. Just like today, I don't want to leave, but I want to. It's all rainy and dreary but I am going to get out of the house. It's good for ya.
I hope you are ok, and don't beat yourself up about feeling down. There may not be an obvious reason, but you just are and that's OK. Don't force yourself to do anything though, just be if you want. But know that people do care, and I hope you have a nice day.