I also started experiencing a similar problem with my linux setup (i.e., serial port being grayed out in the IDE, making it impossible to upload sketches), after a couple of days of being able to upload to my arduino uno just fine.
I was up until 4am trying various solutions and suggestions I read about in other threads and boards to no avail, until I noticed that steps for flashing the firmware involved shortening two pins on the board. Simply performing this step (basically just step 5 of this post, or part of the steps from this page ) worked for me:
- with the USB cable plugged in, locate the two pins closest to the USB port on the board.
- briefly touch the two pins with a piece of wire.
- after the LED stopped blinking, unplug the USB cable from the PC, and plug it back in.
After this point, the Serial Port was detected again in the IDE, and I was able to start using it like before.
I'm still new to this whole thing, so I have to admit I'm not entirely sure if this makes a whole lot of sense (why it stopped working in the first place, and why the above step fixes the problem). Just thought I'd share my experience in case it helps someone out there having a similar problem...