Eclipse autocomplete not working

The hot key combination ctrl+space might be conflict with other settings in system if you are using windows.

Try modify this combination like alt+/, i always use this one since the first time i knew eclipse. It works well.

Hope it works for you.

This is recurring for me. I'm using Eclipse 2019-03 in Windows 10. The steps below work for my case, and does not require a restart.

Window->Preferences->Java->Editor->Content Assist->Advanced

The following options are de-selected, and when I set them the autocomplete worked as before. No restart needed.

 1. Java Non-Type Proposals 
 2. Java Proposals 
 3. Java Type Proposals
 4. Java Proposals (Task-focused)

I don't know what's triggering this but it only happens after I've created a new project. Usually doesn't. It simply doesn't happen often enough for me to notice a pattern to follow up on more. Clearly this problem occurs with a variety of causes, so review the other answers to see if they match your case.

Window->Preferences->Java->Editor->Content Assist->Advanced

The following options have to be selected, then the autocomplete worked . No restart is required.

Java Proposals  (must be selected, not sure the follows)

Java Non-Type Proposals
Java Type Proposals
Java Proposals (Task-focused)