Please help me

My parents rented out our flat because they decided to move back to our village to take care of my grandparents, whose health was declining. Unfortunately, the tenant who moved in has stopped paying rent for months, as well as the maintenance fees. He doesn’t answer calls, and when he does, he falsely claims that he paid the rent earlier—something that’s not true. He’s also refusing to leave the flat, and it’s always locked whenever my parents visit. This is causing a great deal of stress for my parents, and it’s becoming impossible for them to go back repeatedly. I’m unable to assist them since I am in Punjab. I’m heartbroken seeing them so stressed, and I’m not sure what to do :(. Please advise me on what can be done?