Take a look at the sentence again:
Michelle is terrified of spiders, so when she found one in the bathroom,
she panicked, refusing to shower for three days
to use the restroom, she drove to her neighborhood gas station.
The problem occurs between days and to. She panicked, refusing to shower for three days is a complete sentence. To use the restroom, she drove to her neighborhood gas station is the next complete sentence. Because these two parts have only blank space between them, they make a fused sentence, a major error.
To fix the problem, you could put a period after days and then capitalize the T for to.