i received a check in my mail box , and i put it in my account and now my landlord texted me and told me if i cashed somebody else check so ...

Question

i received a check in my mail box , and i put it in my account

and now my landlord texted me and told me if i cashed somebody else check so what will happen next ?

Answer

If the check was made out to you, then fine. But how could you deposit a check made out to someone else who is not you? Why would your bank even let you deposit such a check?

Why is your landlord even involved?

You did a very stupid thing. You need to refund the money but you need to work this out with the person's who the check belongs to before they get the police involved. If you are charged with theft/forgery/fraud, then you cannot do anything and this will have to be worked out in criminal proceedings.