No option to reply to a particular PM

I got a pm from a member and I see no option to reply. Never happened before and I see post reply option for other PMs. What is the reason and solution ?

Some people disabled that function.

When you send a PM you have the option to 'Lock conversation (no responses will be allowed)'. I guess the person who PM'd you must have ticked that box. You could always start a new conversation with them, if you need to continue the discussion.