There are three main credit bureaus (Equifax, Experian, and TransUnion). They provide free annual reports. Check the information on the report as soon as possible.
Make sure your personal information is up-to-date and accurate. Then look at your account information: payment history, account balances, credit inquiries. Make sure old information is not on there.
If you find a mistake, you'll need to file a dispute. First, open an individual account with the credit bureau. Then follow their process. Submitting online will be faster than through mail.
After dealing with the credit bureau, you'll need to contact the lender. Provide information that would be helpful: canceled payment checks, bankruptcy paperwork, account statements, police reports, etc.
Getting rejected is frustrating. Luckily, you can file again and provide more information. You can file a complaint with the Consumer Financial Protection Bureau. You can hire an attorney. You're legally allowed to write a 100-word statement to attach to your credit report.