Computer beeps 4 times and display not coming

Hello,

i was checking my pen drive whether it was booted or not,so I tried to disable legacy boot and restarted.but when it restarts, 4 beep sound occurs and no display is coming,monitor is fine,checked graphics card by switching to integrated from external,reinserted cmos,but still display is not coming,but processor fan,keyboard light are on and normal.what do I do?? pls help..