Can No Longer Add Effects to Video in Premier Pro CC

I have suddenly become unable to drag and drop effects (like keying and many others) onto the videos in the timeline.

This started very suddenly whilst I was working on a project and I haven't experienced this issue before when creating any videos using effects.

I have searched forums and even had Adobe fix re-installation issues with my Creative Cloud but it still has the same problems.

When I drag the effect the little arrow has the no symbol on it as well.

Please Help! I am a student whose project heavily depends upon being able to use this software!

Thank You,

Alfie.