Usually this is just the battery has gone flat. It is quite easy to change, please refer to your user manual, simply remove the lid and the Philips head screws holding the top compressor cover in place, on the MT45F-S and MT35F this includes the hinge screws. Then gently turn over the end cover and you will see on the back of the temp display a round cap, to remove turn anti clock wise with a flat screw driver bit, and you will see the battery inside, remove and replace with a new battery. Replace cap, cover and all the screws and you are done. There have been two types of battery fitted, once again please refer to your user manual or the label on the inside of your lid for battery selection, LR44, or CR2032.