If the tank's temperature drops below 60 degrees Fahrenheit, it's not as urgent as overheating, so you may safely remove the leopard gecko while investigating. If everything is in order, start by double-checking your thermostat and thermometer to ensure that everything is functioning properly. If it's set too low, then adjust the temperatures accordingly. If that's not the problem, there may be a problem with your heat source. It's possible that the problem is with your source of heat. It's not unusual for something like that to happen. Another potential cause of your tank being too cold is that the rest of your home is, as well. This is why it's not a good idea to put the tank in cold areas such as the basement.