Diesel put in fuel tank

That really should not hurt it to much, but about the only thing you can do is have a mechanic put injector cleaner through it to try to get it start. you might try using choke cleaner to fire it then check fuel pressure with a gauge auto parts rent it.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

You can get it, but it is not really that good and i am just taking a guess as to why it wont start. check to see if you have pressure like sent article. if you only put a half gallon in and then like 5-6 of gas it should not hurt any. check your fuses etc. also follow this link on what happens when it wont start. I am assuming there is no check engine light on as well you might try resetting security system as well if no owners manual look online for your vehicle on how to do it.

https://www.2carpros.com/articles/car-cranks-but-wont-start

Probably your battery is low recharge it and try resetting security system if no owners manual look online for your vehicle on how to do it.