Hi Reema Begum,
Which model of Nest Thermoset do you have? If it is the newest Nest Thermostat (2020), that has a separation line between the two pieces of plastic that make up the face of the thermostat (this is not a crack). The main piece of plastic is the main display lens for aesthetics and is an outer layer of finish to the product. The bottom sensor is a PIR Infrared Fresnel lens that allows Google's sensors to function correctly. This is an intentional design on the Nest Thermostat and not a manufacturing fault. To review the pictures of the product, you may refer to Google's marketing material here
If that is not the case, and you do think your thermostat display is genuinely cracked, please can you share a photo of the thermostat display? Also, where and when did you purchase it and what country are you in?