Push the belt back on the cog. If you can't push it on with your hand/fingers, put the crank pulley back on and use the bolts to draw the crank pulley up to the cog, to push the belt back on.
The pulley keeps the belt from riding off the front of the cog. If you rotated or started the engine without the crank pulley in place, the belt will normally walk off the cog a little bit.