Why does my Vacuum only work if I am holding the bottom down?

This is a problem many times over for any Dyson unit. The switches are almost underrated for the motors. You can try this one trick, unplug the unit and hold the unit facing you . depress the plastic button hard then slip your fingers off it to let it snap up quickly. Several actions of this may well get the switch to toggle once again. If no luck then replacing the switch is easy. I should say there is two switches one for the beater and one for the suction. If you go to e-bay and enter your model number you will find your switch to be about $5.00. There is a black one and a red one. I would get both and replace both while you have a chance. Utube has several movies on how to disassemble the handle. So take a breath and take your time and your unit will be back working again. Good luck