there is no guarantee of success.

Shut down the Mac and then fire it up.

Hold down the left mouse button at the chime. Keep it down.

If you are lucky, one of the disks will move enough for you to catch it with needle nose pliers or such.

It is likely to destroy the disk but that is cheaper than getting someone to dismantle your iMac to remove the optical drive and then the disks.

Also be aware that there is a possibility that the drive itself will be damaged already.

Good luck
Let us know how you get on