SIM800L project not working
The module needs a good 5v supplier as it might have a peak of 1A or 2A, when it registers to the network or connects to 3G.

You need to make sure that you share the ground between the power supplier and you use some 2 pins on your arduino to connect the GSM module.
Dont try to get a fast baudrate as software serial doesnt work, try with 9600 to start then see if it response to the AT command.

SIM800 is a very simple module, try with an FTDI in case you want to make sure you didnt break your Arduino.

For more help, share your schematic how you have connected the arduino.