Have an Uno hooked up to my laptop. Successfully selected board type and port (only one port available). Uploaded Blink program to UNO to verify proper connection; worked correctly. Completed project to blink external LED (new to all of this and just getting my feet wet); worked correctly. Threw a slightly more complicated build and sketch together, would not load. Went to check COM port and the Port option on the Tools menu was greyed out and could not be selected. Disconnected and reconnected UNO, was told it was an unrecognized USB device. Any idea on why it would work initially and then suddenly become unrecognized? Any suggestion on how to fix this? Thanks for helping a newbie out.
Remove anything you have connected to the UNO and try again. You may have a short circuit on your project.