If you aren't connected to your PC (with the USB cable) and your Kindle is still stuck on the USB screen and you can't access anything... hold the power switch over for about 30 seconds and then release it. That should restart the Kindle.
If you're still connected via the USB cable and it won't "eject" properly, then that's a different issue. I seem to remember there was a slightly different process involved on Windows 7 than there was with XP/Vista. Unfortunately, I can't help with Win 7. It's not really a super-duper big deal if you just unplug it.
I find that I very rarely need to be able to read or access my Kindle's menus while it's plugged into my PC anyway. I'm either reading, charging, or transferring and rarely do those intersect for me
