Your question, by what, shows why it's not passive. In this case, "exhausted" is an adjective, and thus the sentence is not passive voice. The subject (I) is not receiving the action of getting used up or drained, it is doing the action of being in a drained, used up state. Now, if the sentence was something else like "I was exhausted by his tiresome ramblings," then yes, that would be passive voice.
korethr|9 years ago