Q: engine turns over but won't start on 1998 Jeep Wrangler

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My girlfriend was driving her jeep when all of a sudden the engine died. Now the engine turns over fine but no sign of it starting. Heard about replacing crank sensor but it's 70 dollars and didn't want to spend the money if theres something else i can try. Also a day before this happened she filled up with gas and never put the cap back on. Thought air might of gotten in fuel line but very unlikely seeing how it was full of gas. I checked bleeder valve on fuel injection rail and after bleeding and cranking always spits out fuel.
(1) Answer
To avoid spending money on parts that aren't needed you'll want to perform a little diagnosis. Start by checking fault codes in the engine control module, you may have a diagnostic trouble code (DTC) stored, this code will help point you in the right direction.
Your concerns about the gas cap and air in the line are not likely an issue.
If you get the code read out, perform a search here and we have a ton of info about the most common DTC's.