The string is broken, you do not have the spare one, or don't want to spend money ...
Step 1: Unwind the String So You Have Enough Length to Work with. 5cm - 10cm Past the Bridge Is Enough. Optionaly You Can Completely Remove the String, as I Did in the Clip.
Step 2: Protrude String Through the Eye of the Ball, Bend It and Hold All 3 "Wires" (2 of Bent String + 1 Short Broken Piece Attached to the Ball) With Pliers So That When You Twist, You Twist All 3.
Step 3: Take the String Ball with Another Pliers, and Twist CW, or CCW Until You Get Nice and Fine Twisted Wire Looks, Even More Important, Untill It's Firmly Twisted Together.
Step 4: Put the String Back and Tune Up.