Do you know anything about depression in people? 
Depression is rough for sure -- I have personal experience with it. It can actually be a symptom of some diseases or a side effect of some drugs.
I experienced it after taking a very strong antibiotic. I didn't know that could happen! 
How did you feel? There's such a wide range of effects -- sadness, anxiety, emptiness, hopelessness...
I realized I was depressed after sleeping when I should have been working. It lasted more than a week. 
Sounds like Major Depressive Disorder. How was your self esteem? It can significantly reduce it.