I cant break blocks or hit mobs. I have tried eveything to fix this but nothing.

In a (new or old) world, I place a block which works perfectly, and when I try to break it I cant. I also cant hit mobs nor kill them. I know I didn't change the setting for placing blocks and I know my mouse works perfect. I look at youtube videos and this isn't happening to any one else. I tried downloading a new minecraft but Its just the same.

*What I expected to happen was to break any block

*What actually happened was nothing

Steps to Reproduce:(which seems to be only me)
1. Place block
2. Look at the block
3. Left click which does nothing