The simplest way is to look into the distance. If you feel the things are blurred and your eyes are still comfortable, your contact is fallen out. If you feel the things are blurred and your eyes are not comfortable, your contact may move into your eyes. The second way is to touch your eyeballs with your hand gently. Remember, you must wash your hand clean. If your eyes don't have any feel, your contact must still in your eyes because it is the contact that touches your eye. If your eyes blink as a kind of conditioned response, your contact must have fallen down because your eyes will response to the outside touch automatically. If sleep without taking off the contact lenses and they moved into the eyes, you may need to see a doctor to take them out.
Well, sounds like you haven't been wearing contact lenses for a long time. Anyway, if you have myopia and you are with a pair of contacts, your world would be much clearer and brighter, however, as soon as one of them fall off your eyes, you would feel blurred images in front of you. So you should start looking for it and afterwards thoroughly clean it up.
You can close each eye and concentrate on a single object with the other to see which one has been affected. In this procedure, blurriness in one eye indicates that the lens of that eye has dropped. Look into a mirror to check the affected eye to locate your contact lens, maybe it is away from the cornea. Then wash your hands, slightly rub the eyelid to get the lens back over the cornea. If you have difficulty in this procedure, use eye drops to loosen the lens. After having removed the contact, check if the lens is in correct bowl shape. If so, you can wear them.