Help! Father's Social Security suddenly suspended

Hello! I had a question about my father's social security. He is 83, and has been receiving ~2k monthly for retirement benefits. He's told me that he did not get his deposit this month, and when logging into his account, it tells him that his benefits are suspended - though I am not sure why.

However, my mother (a housewife and his dependent), receives ~$900 monthly, and she did receive the deposit this month.

Context - they live outside of the US, and all correspondence goes to a USA P.O. box, which accumulates and gets mailed out to them every month or two. So we're not sure if they were mailed any letters from the govt. They have not worked in 20 years, they have not moved in the last 7 years, and to my knowledge, there have been no major changes in their life.

Does anyone have any advice? Or why this would have happened? Thank you!