I had the same thing happen towards the end of my last project. Not only could I not reimport a clip I deleted but any new media wouldnt show up as well. A new event solved the problem but it makes me nervous about the stability of FCP X
If you want to dig deep I recommend downloading autodesk smoke or after effects. Smoke is free for the next couple of months and is designed to do what you want…
I have experienced this as well. Not cool when wearing headphones late at night! Not sure how to solve it but so far it only happens at home with out a video i/o. Do you have any cards in your system?