Hi Samrat,
While it's difficult to say with certainty given your rather terse question and lack of information, but the best guess at this point would be that your screen has failed and needs to be replaced. It would be worthwhile to open the phone up and check the two connectors going from the screen to the rest of the phone, but if that doesn't help your best bet is to go ahead and replace the whole screen assembly.
I didn't find a lot of information on how to replace your screen, but there is one YouTube video that shows the process. Please note that this is a decidedly non-professional repair, so use common sense when taking it apart; mark the position of the screws so they go back in where they were remove from and use heat to soften the adhesive on the screen before removing it; an ordinary hair dryer will work fine.
[link|https://www.youtube.com/watch?v=uCHLNSJE29c|Infinix Hot S X521 Disassembly / display replacement lcd - YouTube|new_window=true]
You can find replacement screens on most of the major sites; I tend to start with Amazon, eBay and AliExpress myself, but there will be others as well depending on your location and the availability of parts there. Here are a couple that came up on a quick search.
[link|https://www.amazon.com/CAIFENG-Replacement-Digitizer-Assembly-Infinix/dp/B07WYZLWH9|Amazon.com: CAIFENG Repair Replacement Parts LCD Screen and Digitizer Full Assembly for Tecno Infinix Hot S X521(Black) Phone Spare Parts (Color : Black) : Cell Phones & Accessories|new_window=true]
[link|https://www.aliexpress.com/i/2251832805879325.html|For Infinix Hot S X521 Lcd Display Touch Screen Assembly Glass Panel Digitizer Touch Sensor Replacement - Mobile Phone Lcd Screens - AliExpress|new_window=true]