Ok so the subject title pretty much sums everything up.
Earlier on today the arduino uno was working just fine. I had pin 8 sending out a high signal to the grain of a mosfet which was hooked up to a pump & a 12V power supply (Drain). The source was grounded.

Then, the Tx, Rx, and L (pin 13 led) turned on and now every time I plug the arduino to the usb drive on my computer the three leds stay on.
When I try to upload a program I get the following error:  Serial port 'COM6' not found.
And I checked all the pins with a voltameter and I found out that they were all 3.8V.

What is going on?