I remember quite a bit of it but not the title or author. Humans were visited aeons ago by a race that went around suppressing psychic development of young species as they were unable to develop these themselves. In modern times another race sends a weapon as a precursor to invasion but the weapon unlocks the humans' psychic abilities and people suddenly are able to alter reality at will, the main character uses the trigger word Aufmachen to stop himself accidentally affecting things.
Blindfold from the Stars by Philip E.High
The conquest of a galaxy is relatively easy providing the aggressor has the necessary technology and, more important, a safe method of overcoming possible opposition without bleeding to death in the process. The Asdrake employed mutated micro-organisms which directly attacked the brain of the planet's intelligent life - they liked this method. They could fight if they had to but were inherently lazy. This method had worked successfully on twenty-two occasions - why should it fail on planet 5/6/9. Sector 88. Sun System 46. "Tseudec" (Native name: "Earth").
Casualties from the epidemic were astronomical yet, in complete paradox, failed to kill a single human being - the side effects took care of that. And those side effects were beyond even the Asdrake's imagination. The survivors on Earth found undreamed complications in rebuilding the world with the use of their new psychic powers - and perhaps more important, were they new powers at all, and if not who had removed them thousands of years ago?"