Is my cat in shock? He is acting very strange.?

My cat has not been himself lately. He spent all day outside yesterday (unusual for him), and now all he does is lie under my bed and stare into space. He sits very still, rarely gets up, and doesn't purr when I pet him like he usually does. When he walks he is very slow, but doesn't seem to have any visible injuries or wounds. Is he in shock? Should I take him to the vet? Is there anything wrong with him??