SIM800L project not working I got a SIM800L, same as the one under this link, Quickstart SIM800 (SIM800L) With Arduino, and played around with an Arduino Nano and Uno. I had a few problems, at the start, not getting it work properly. It turned out that my Nano 5V pin wasn't enough for the SIM unit. It works fine with Uno though.

There is the code I tested the GSM (same link) and got it working somehow, I got simple AT commands and managed to send an SMS. But after I dismantled the connections and the next day I tried again... Nothing. I can see the SIM unit connects to network as the LED blinking slows down to once every 3 seconds but when I type commands in nothing happens in the serial monitor. It doesn't give errors or anything, just nothing happens on the serial monitor.

I did wire it incorrectly, once with the Tx/Rx, but that shouldn't damage the unit, should it? 

I tried to upload different programs to my Arduino and they work fine so that shouldn't be the problem. Is there a way to reset the SIM800 unit? Or am I missing something here?
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.