This is a fairly superficial question, but: tonight I dropped my N900, and off popped the back cover and the battery. The phone still works fine, apart from the window over the camera.
The window over the camera is now pretty rigid, and takes a bit of effort to open and close. I think the spring has been dislodged; I don't see an easy way to fix this.
So, are there any spare back covers kicking around?
Have you lost the magnet from under the kickstand?
I stupidly coloured the inside of the camera slider in black to try to get rid of the blue hue, only to realise the camera uses the black/white blocks on the inside of the slider to register whether the lens cover is closed or open, I then spent the next 45 minutes wiping off the marker pen live and learn ay.
Maybe you've changed the distance between camera and lens cover and therefore the camera is not registering the lens cover being opened.
Might be worth taking the back cover off and pushing the lens cover from the inside out, see if you can click it back into position.
This is all a bit of a guess so don't flame me too much if it doesn't help.
You can get replacement back covers, I have the link somewhere and will update this post when I find it.
Oh, actually I fixed it. I popped off the lens cover completely. There's a pin which hooks into the backplate; the pin slides back and forth, giving the nice spring-loaded feel you get when you open or close the cover.
That pin had jumped out of its socket, and so it was scraping across the external plastic cover. Fixed it now, thanks to a bit of perseverance!
live and learn ay.