my cd is blocked in optical drive ! i can't unlock

Try powering off the computer, then restarting it while holding the left mouse button down. This is the universal command to a Mac to eject the media in the built-in drive. Hopefully this will release the disc for you.

If not, it is possible the optical drive has malfunctioned - a common issue is the mechanical safety block failing to release. This arm comes down after a disc is loaded, preventing the accidental insertion of a second disc. Shine a light into the optical drive slot - you should see it easily. If necessary, you might be able to help the blocking arm back up by carefully using a flat blade while the drive is attempting to eject the disc. Doing this, however, may damage the disc, and definitely indicates the drive must be replaced.

If you still cannot get the disc to eject, you will need to disassemble the optical drive (4 screws hold its top cover) to remove the disc.

You haven't specified the exact model iMac, so removing/replacing the optical drive may be a trivial matter (iMac G5 1st/2nd gen), or incredibly involved (all other iMacs).