I had exactly the same problem recently, right after having upgraded my Eclipse.
Open your preferences (Window/Preferences/Java/Editor/Content Assist/Advanced) and ensure that "Java Proposals" is checked:
If this was fine already, check the level above (Window/Preferences/Java/Editor/Content Assist), as follows:
EDIT (as commented below) Should both settings be correct, Eclipse is maybe a little overwhelmed. Try to restart it and wait for the worskpace tasks (build, compile, highlighting, ...) to finish properly. It often works.