A lost ATV key can be a frustrating experience. All modern ATVs require a key whether they have electric start or not. Without the key, there is no easy way to start the ATV. The key must be replaced.
Determine your ATV model. Write down the exact model ATV you have. Include any and all information you can find.
Find your VIN number. Just like a car, an ATV has a VIN (Vehicle Identification Number). In most cases the number is stamped into the frame, or written on a plate that is riveted or welded to the frame. Write down this long number as it will have most of the information you will need.
Call or visit your dealership. Even if you did not buy the ATV new, you should be able to find a local dealer that sells the brand you are working with. Call that dealer and tell them you need a new key cut. With the VIN number and model type, they should be able to cut you a new key that will get you up and moving. This is not usually an expensive service.
In the event that your ATV is too old, or you cannot find a VIN number, you may have to replace your ignition switch or have your dealer do it. While not a simple answer to the problem it will work.
Do not try any "hot wire" tips or ignition bypassing on modern ATVs. You can do costly damage to the electronics. Also, many ATVs are equipped with antitheft ignitions that make "hot wiring" them impractical.