URL getting changed

I am facing a problem with my website’s URL which cropped up recently

Whenever I am trying to go to particular URL eg https://www.mysite.org/what-we-do/ I am getting re-directed to https://www.mysite.org/?what-we-do%2F and this opens up my frontpage . My permalink structure is https://www.mysite.org/sample-post/ . I tried disabling all my plugins but that didnt solve the problem . Any idea what might be causing this and a good starting point to troubleshoot this ?