What does it mean when my dog is sad and shakes a lot?

He has been moping around the house since yesterday and i found him in the basemennt laying on the ground shaking. As i tried to get him to come up the stairs he limped up the stairs, he is only about 6 years old. Also when he wal;ked into my room his tail was between his legs and he looked very weak. What is this shaking? He also has food but is not eating it.