You didn't say so, but I'm assuming you have a slot-loading optical drive.
That being the case, you've also got something of a problem. Since it isn't mounted on the desktop, it's probably just rattling around in there, so your best bet is to turn the computer off, tilt it so the drive is facing down, and shake, hoping the disc comes out. If that doesn't work, it's either stuck, or there might be a little anti-dust flange holding it in, in which case you can try getting someone with a flashlight and maybe a straightened paperclip to help you dig around in there, but you're at serious risk of breaking the drive entirely if you poke or scratch something important.
That, or take it in to a repair place, ask them to remove it, and hope the drive's not broken.
Mini and "business card" CDs just don't work in slot load drives, and though I have seen adapters for the small CDs, they're probably more likely to break your drive than not, so I'd just give up. If it's hugely important, go find a cheap external tray-loading drive--you can get a 5.25" USB2 or Firewire case for $30 or $40, and even a top of the line read/write everything drive doesn't cost more than $50 or so. An old Iomega USB1 CDRW can probably be had for $20 on eBay.
