Why is there no display?

@surayakhatun could be an issue with the display, the BIOS, or the motherboard.

Start off by connecting external monitor. See if that displays properly. If so, you know you have a bad display.

TO see if you can reset the BIOS, turn off the laptop and disconnect the charger. Remove the bottom cover from the laptop and disconnect the main battery cable from the motherboard. Disconnect the RTC battery from the motherboard. Once the RTC battery is removed, measure its voltage. If the voltage is less than 2.6V, replace it. Here is the Lenovo IdeaPad 3 14IAU7 Hardware Maintenance Manual that will help you with those tasks.

Next, press and hold the power button for 30 seconds and then release it.

Now reconnect the RTC battery and reconnect the main battery; then try to turn on the laptop and check what happens.

If none of that worked, you may have issues with the power circuitry on your motherboard. For that we would need to see your board and to know what exact model your motherboard is. Post some good pictures of that with your question. That way we can see what you see and hopefully help you out further.

Adding images to an existing question