Some common causes for a broken touch screen on a cellphone include sharp twisting, a hard impact or even pressing hard enough on the back of the phone. Designers have made them to put up with the stresses of regular use, but the screens prove fragile under impact.
In some cases, the broken screen only shows cracks, as though a spider spun a web across the front of the device. Running a finger across the surface reveals the presence of cracks. This can leave small glass slivers in the skin, so it's not recommended. In other cases, the damage renders the screen completely inoperable. The web-like cracking is more common in shorter drops, and the complete disability results with a higher drop or in cases of something striking the screen.
In other cases, the glass doesn't break completely, but the screen looks like it has a huge blot beneath the glass, with trails running out like the tentacles from a jellyfish. This usually happens when excess pressure on the front or back of the phone has gone beyond what the screen handles. The chemical inside the screen that renders images has developed a leak, and under enough pressure, the glass shatters, and the chemical comes out.