Code Completion Keeps Auto Turning Off

I don't know whether I've pushed a hot key to turn it off, but it's checked on in preferences.
I can't find any answers online.

I have tried restarting the program.
Tick/unticking the tick box.

Solved.
I restarted my pc.

Update:
This keeps occurring, It happened twice yesterday, and it has now happened again today. I am having to restart my pc to get the Auto Complete Code to work again.
Has anyone had a similar issue?