Here is the item 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.
Charlie used two fingers to hunt and peck out the 2,000-word research paper is a main clause. When lightning caused his computer to crash is a subordinate clause that begins a second main clause, Charlie wept .... Allowing the words paper and when to collide with no punctuation creates a fused sentence.
To fix this error, you could put a period after paper and capitalize the W of when.