The Upower Up-200 nail drill is essentially a dremel tool that's meant to be used on your nails rather than in the workshop. The base unit is a generator with a variable rpm input, and it is connected to the hand-held unit. If your nail drill starts giving you trouble, you might be able to find and solve the problem yourself if you go down a basic checklist.
Examine all of your settings on the base unit to be sure no switches were flipped or moved when you weren't looking. Make sure the unit is plugged in and that you have power from the plug that you're using. Once you've made sure the settings are correct and have eliminated outside problems like no electricity, you'll need to pinpoint the difficulties with the unit.
Check the electrical cord that connects the unit to the wall, and the cord that connects the base unit to the hand-held drill. Problems in the cord like cuts, nicks, stripped insulation or even broken wires can be a cause for shorts, lack of power and intermittent functioning.
Examine the bit and the chuck. If a bit is not firmly held by the chuck it won't spin properly. Tighten the chuck by turning it by hand until it grips the bit.
If the bit is worn down, or if it has not been cleaned in some time, it won't work properly. Wash the bit and examine the roughness. If necessary, replace the bit with a new one.
Check your owner's manual for further troubleshooting procedures if you cannot locate the problem and fix it by following the above steps.