Take a look at the sentence again:
Hunched over the keyboard for hours, Charlie used two fingers to hunt and
peck out the 2,000-word research paper when lightning caused his computer
to crash, Charlie wept more tears than the
thunderclouds spilled rain.
When lightning caused his computer to crash is a subordinate clause, so it cannot stand alone as a complete sentence. Charlie wept ... begins a main clause. It is both legal and necessary to connect these two items with a comma.
Go back to the sentence to try again.