Why MY pc unable to turn on?

What happens when you press or hold down the power button in the Top Right corner right of the delete key above the backspace key?

Is the computer charged?

If you hold down the power button and the screen turns on OR the keyboard turns on then the computer should be on. If it seems to be on but it only shows the Dell logo or (screen on/keyboard on) then press the "F2" key at the top of the keyboard repeatedly until the bios screen opens. Then from there, I'm not completely sure on the correct way to help your problem but resetting the BIOS might help.

Steps the reset bios (to default, not resetting the CMOS or NVRAM) (from Dell website)

1. Turn on the computer.
2. Tap the F2 key several times until Entering Setup appears.
3. Reset the BIOS to factory defaults. The method to reset the BIOS varies between different computers:
   • Click the Load defaults button. OR (I think this is correct for the latitude 3420)
   • Press the F9 key to load the default settings. OR
   • Press the Alt + F keys to load the default settings. OR
4. Press the Esc key and select Save and Exit or Exit.
5. Press the Enter key to save all changes and exit the BIOS setup screen.
6. The computer restarts and boots to the operating system.

This will help

for your device at iFixit. Whatever you do, please be sure todispose of your battery properly.

Dell Laptop Won't Turn On - iFixit Troubleshooting

Also, the fix I showed earlier "reset's" the Bios but DOESNT reset the CMOS or NVRAM, so
How to clear the BIOS, CMOS, or NVRAM on Desktop computers without an RTCRST Jumper or serviceable Coin Cell battery

If your desktop computer was produced before April 2020, the computer most likely has a jumper-based reset. Desktop computers manufactured after April 2020 use the RTC Reset ability.

Note: Desktop computers released after April 2020 should have RTC Reset enabled by default.

For desktop computers that use RTC Reset, hold the power button for 30 seconds.