Latitude E5540 Keyboard not responding

I have a Laptop Latitude E5540 that I dropped it on the floor. After that everything worked fine for 2 weeks. After 2 weeks the keyboard started not working, I have to press really hard to type, but now there are some keys that wont even work. I installed an external keyboard and it works fine. 

Here are my specs: 

Latitude 14 5000 Series

Windows 7 Professional, Canada MUI, 64bit (includes Windows 8.1 Pro 64bit License and Media)

4GB Single Channel DDR3L 1600MHz

500GB (5400 rpm) Serial ATA Hard Drive

I installed a keyboard update from dell and updated the bios. Also, I already exchanged the keyboard for a new one and nothing changed. Any suggestions please. 

Thanks,

Ricardo