I am new to this so sorry if anything sounds too obvious...

I just recently finished building my quad. everything seemed to be working and I was able to bind my radio and everything. but, just as I was finishing up configuring in beta flight the quad started beeping. I unplugged the battery and plugged it back in but now I only hear 3 start-up beeps instead of the previous 5.

in addition to that, there is only 1 green LED lit up on the FC instead of the brightly lit white and blue ones there were previously. also, the battery voltage icon in betaflight says I'm only getting about 4.3 volts when the battery is connected. I tried to connect a fully charged battery and there was no difference. 

could this be a problem with my FC or battery connection???

please let me know

thank you.

There's a possibility that one or more of the onboard voltage regulators are fried. The 5 Status LEDs on the Mamba F405 MK2 flight controller are indicators for the following:-

1. Blue -> Betaflight (or other firmware) Status LED
   
2. Orange -> Betaflight (or other firmware) Status LED
   
3. Green -> 9V Power Rail Active
   
4. White -> 5V Power Rail Active
   
5. Red -> 3.3V Power Rail Active
   

So with no white or red LEDs it sounds like you have lost the 5V and 3.3V power rails. The 3.3V power rail usually has a dependency on the 5V power rail, so if the 5V power rail dies then the 3.3V power rail also gets starved of power.

With no 3.3V power rail active, the MCU won't power on and thus Betaflight won't be running so you won't get the blue or orange LEDs either.

Try plugging in a USB cable and see if the 5V rail then becomes active.