I can't hit mobs unless I use a tool.

I was throwing snowballs one day at mobs in one of my creative worlds and when I tried to kill it with my fists I couldn't hit it unless I had a tool like a sword or a pickaxe out. I also tried destroying a painting but I couldn't unless I clicked on it with a tool. This also happened to me in one of my survival worlds so I had to delete it because I couldn't hit any mob even with a tool. Please fix this bug because I worked really hard on building my creative world.