Washington Redskins (Super Bowl XXII; January 31, 1988)
Falling behind 10-0 (2-0?) to the allegedly unstoppable Broncos (Cavs?), who were led by suspected diety John Elway (LeBron?), the Redskins nevertheless stormed back to stomp the horses 42-10, the first time anyone had come back from 10 points down to win a Super Bowl. Take note, Cleveland (though the analogy of playing in a Super Bowl may be lost on you).
Abraham Lincoln (1864 Presidential Election)
Things looked bleak for Abe in the summer of 1864, with citizens becoming increasingly weary of the Civil War as Northern casualties (unlikely injuries?) rose at a staggering rate. Calls for peace from oppossing candidate George McClellan seemed to be catching on and Lincoln's re-election, by his own admission, appeared to be a long shot. But he persevered, eventually winning in a dramatic landslide and redoubling his efforts to preserve the Union (loyal fanbase?). Honest Abe, as we know, lived in Washington at the time. Where was McClellan's running mate, George Pendleton, from? Ohio! Think about it.
Lazarus (approximately 29 A.D.)
One of the most stunning comebacks of all time, he was literally dead and buried (written off by the local media?); much worse off than having a 2-0 first-round series deficet. Still, Laz pulled off the upset, unknowingly leaving an inkling of hope for Wizards fans almost two millenia later.
Romans (Second Punic War; 218-202 B.C.)
The Romans were reeling after Hannibal and his Carthaginian army crossed the Alps and tore through northern Italy (took the first two games in arguably convincing fashion?). However, Rome held it together, stemmed the tide, and eventually moved the battle to Carthaginian territory, sealing their victory at the Battle of Zama on the outskirts of Carthage itself (took game 7 on the road?).
We could go on, but the above should be sufficient evidence to convince even the staunchest optimism-haters among us that a stunning comeback victory is not only possible but, given these lessons of history, probable.