Each of the women in the Mima family demonstrated powers resembling telekinesis. With each successive generation, those powers increased. Tomiko's (Key's mother) powers surpassed those of Toyoko (Key's grandmother) just as Key's abilities far surpassed those of her mother. Concerned for Key's safety, Dr. Mima injected Key with the Gel he had collected from Tomiko during his years of study, with the result of "sealing" Key's soul--the core of her powers--from the physical world and most notably from Ajo.
This also explains how smaller groups of people who emote on Key's behalf can cause Key's abilites to surface: her soul is freed from its Gel prison --albeit temporarily--and her true self emerges, along with her abilities. After a time, however, the Gel of her mother re-asserts itself and closes off Key's soul, precipitating a blackout.
Notice how the only things in the picture that do not appear to be moving are Key, Dr. Mima and the collapsing tower? Also see how everyone is running away from the tower and the unbalanced aspect of the drummer standing on the tower?
Key telekinetically stopped the tower's collapse until her grandfather could be rescued. It was the first demonstration of how powerful her abilites truly were.
In what is sure to be *the* most depressing part of the entire series, Sakura dies in her own apartment shortly after the rescue. Too much had been taken from her and her body expired from the ordeal. One thing a lot of people won't know about is that as Sakura begins to slip into her final sleep, Key says to her "Sakura, don't fall asleep--if you do you will never wake up." These same words are the very first Key ever spoke to Sakura when they first met in junior high school. Sakura was falling asleep at her desk. Key, having been told by Dr. Mima "never to fall asleep or you will never wake up" in her childhood (this deals with the Gel injections Key got at that time), believes these words to the point she thinks they apply to others as well. This is why Key never sleeps (except during blackouts, when her soul is submerged into her mother's essence once again).
The one point of hope in dealing with Sakura's death is that Key found Sakura's Gel cannister and--this is my guess, mind you--injected Miho's dying body with Sakura Gel, saving both her old friend and her new friend.