A few things you should take care of. I can tell you were sold something from the pet store because the husbandry is totally wrong for a leopard gecko.
1. Leopard geckos require belly heat, so you will want to purchase an under-tank heating pad. You want to buy one that would cover 1/3 of your tank, and stick it under the tank. I have an example of how you can get it setup
correctly here.
Leopard geckos in general don't like bright light, they are crepuscular or nocturnal, so they come out at night only. While basking lights can work, I really wouldn't recommend using them for a novice gecko owner. So I would get rid of the day light, and purchase an undertank heating pad. You can keep the red light for night time viewing purposes if you wish, that's optional. Leopard geckos do not require basking light during the day, they'd much prefer to sleep during the day.
*EDIT* Just read more carefully you do have an under tank heating pad, so you should be fine once you switch out the sand.
The ideal temperature on the ground where they lay on needs to be between 90-95 degrees. If your temperature is only 80, it would make perfect sense why your gecko is not eating or moving around. It's too cold for them. To test the temperature properly you would need to use either a digital thermometer with a probe, or a temperature gun. Those stick on thermometers only tell you air temperature, which isn't as important as knowing the belly temperature where they lay on. Buying an undertank heating pad should take care of the heating needs for them.
2. Sand is bad, it's a loose substrate that could cause impaction. Impaction would make your gecko not want to eat, and even go as far as dying due to it. I know there are other leopard gecko owners that do use sand, but again, it's never something I would recommend. Why risk it when you could use much safer, cleaner, and better looking slate/ceramic tiles? So first thing I would suggest is to take the sand out, completely clean the tank out. Then put ceramic/slate tiles in there, or paper towels if you can't buy some tiles just yet. You will feel better knowing your gecko will not ever get impacted by loose substrate such as sand.
3. When feeding crickets, never leave crickets in the cage for longer than an hour. Always observe the feeding, and if the gecko does not eat the crickets, take them out. Crickets can cause your gecko stress, they can also injure your gecko by biting their soft spots like their toes & eyes. Stress can cause your gecko great harm, it's one of the #1 causes for them to not eat or have a shorter lifespan.
If you take a picture of your tank setup, folks here can help you with suggestions and advices. Good luck. The good thing is you are here asking questions, that's always a great first step
