A few solutions.
First, try restoring the PS3 OS.
http://community.eu.playstation.com/t5/Technical-Help/If-your-PS3-is-not-reading-disks-there-may-be-a-solution/td-p/6086723
Alternatively, if you're not worried about losing your saved data, or can store it online/transfer it off somewhere, you can attempt a full factory default reset blah blah, which is doable via the same menu as the method above.
If you're still under warranty, you can ask for a new one. If not, you can trade it in for an old refurbished one for 150 dollars or something.
If you're not down with that, you can try and fix it yourself. Chances are the lens for reading discs is either dirty, or broken.
These will void your warranty.
You can try to clean it by following guides like:
http://www.youtube.com/watch?v=7O-8NzYHr80
If that won't cut it, you can buy a replacement lens for 50 dollars-ish on amazon or ebay. You'll have to install it yourself, it apparently is not TOO difficult. Just make sure you purchase the right kind of lens, because there are two different ones and they're compatible with different PS3 versions.
http://www.youtube.com/watch?v=A0_-n3bLWiM
My brother gave me his old (broken with this issue) PS3, and I was able to fix it just by cleaning the lens.
Of note, if you decide NOT to do any of this and just buy a new PS3, apparently the "heavier" models have this problem less often. It seems to be more common with the 40G version of the PS3.
Hope this helps, sorry there's no really easy fix.
