L and ON keeps blinking
You should split your problem in pieces. Like you cannot get communication with your Arduino again, so check first:


If you plug in the Arduino / USB, do you get a message on your computer?
Is it visible in the systems manager?
Is it visible by the IDE?
If you connect an adapter (12 or 9V), does it make a difference?
If you try another Arduino, does it work like it should be?


If you are a beginner, always make sure:


Never change any wiring with the Arduino on
Never have dangling wires, if you do not need a wire for the time being, either remove it completely or put it in a breadboard hole that is not used
Never change components with the Arduino on
After rewiring/adding/changing components, double check first


Update for comment below because it does not fit as comment:
To buy a cheap/fake (but working) Arduino uno, e.g. 
Arduino Uno from Aliexpress