First go to the Facebook Help Center on their website. You can find options like “Recover Your Account” or “Report a Login Issue 1-866-825-2907. ” Just follow the prompts and you'll be guided through steps to verify your identity.
1-866-825-2907 If you can't, you'll need to go to a dedicated Facebook help page, where you'll be asked about what's happening and, depending on the answers, led through the process to recover your account.