Review the original item:
Running through the parking lot, Aisha hoped to get to her car before the
rain began when she found the doors locked
and saw her keys lying on the front seat, she knew that she was in for a
soaking.
You wanted to fix it this way:
Running through the parking lot, Aisha hoped to get to her car before the
rain began;
When she found the doors locked and saw her keys lying on the front
seat, she knew that she was in for a soaking.
Using a semicolon is an excellent way to join two complete sentences. However, when you use this method, do not capitalize the word that follows the semicolon (unless it is a proper noun that is always capitalized).
Because the When is capitalized, this choice is incorrect.
You might want to consult the rules for fixing comma splices and fused sentences.