Not able to upload any code to Arduino Uno NOTE: Using the Arduino Uno

I am WORKING on a project that I have to submit in a few days. My arduino was all set up and was working fine. 

Project:Arduino controls a 9v motor depending on the command given. As the arduino only provides 5v , i added a 9v battery to it through a transistor

I decided to add new code into the arduino but when I tried it failed. I have uploaded code thousands of times before (successfully) all of a sudden the IDE keeps giving the following message


  avrdude: ser_open(): can't open device "\.\COM3": Access is denied.


I have looked up for a lot of solutions on this forum and elsewhere. Here are the things I have tried so far:


Changing the COM port 
Disabling all the other devices CONNECTED to COM
Tightening the CONNECTIONS
Re installing all the drivers and the IDE
Rebooting the system
Using another laptop to upload code


All the above have given me the same output for some reason. The built in LED light and the power LED switch on when the board is CONNECTED to a power source (my laptop). Could the board have been damaged by any chance ? How can I check if the board is spoilt ? Should I buy a new board ?
I have a post Have I bricked my Arduino Uno? Problems with uploading to board - that describes various tests you can carry out to isolate where the problem is.