What is the ruling on someone who performed Hajj and then forgot about the prayer for a day or more, or was late for it? Is there a supplication or something to alleviate that?

Prayer is a grave matter, and it must be observed at its appointed times, as Allah the Almighty says: "Indeed, prayer has been decreed upon the believers a decree of specified times," and "So woe to those who pray, [But] who are heedless of their prayer." If you suspect that there are prayers you have not performed on time or made up, and this suspicion is based on evidence and not mere whispers, then you must make up what you suspect to clear your conscience; for caution in matters of religion is required. There is no specific supplication for forgetfulness, but you can supplicate to Allah to remove it from you. Focusing and paying attention to worship, especially prayer and performing it at its appointed times, helps to alleviate forgetfulness.