So, for some strange reason halfway through a session I was completely unable to deal damage/heal my players at all. 
the option is just completely greyed out in the combat tracker of the encounter. did I click something and am an idiot or what?? I can't find any discussions about this. 

Those numbers are controlled by the players adjusting their characters’ HP directly on their own sheets.

But I used to be able to change their HP myself as DM when I attacked as monsters and now I can't, which is odd. why did it change? 

Because they hadn’t linked them to the character sheets yet before. The demand for that was overwhelming. No, before you ask there is not switch for it and pro’ly no plans to add one.