JVC MX-KB30 will not read CD's. One minute it was working. Then it quit and says no disc even though there were 3 discs in it. I tried changing the discs, turning it off and on.

Are you closing the disc? If so are you using CRD or CRDW. If you are using CDRW it will not burn in the proper output format. If you use CDR only it should. Also try recording at a slower speed. It has helped me in the past to do so.
A great program to try is CDBurnerXP, and it's free. After you record it then put it back in and run it again, but this time just select "Disc" then from the drop down menu Disc Information [CTRL+I] and read what format you ended up with. Remove that and check a store bought disc and compare.

CDBurnerXP Free CD and DVD burning software

You could try a CD lens cleaning disc which will clean any dust off the laser lens. But it is possible the laser power can be low if it is an old player. this will stop the discs being read fence the fault No CD as it cant see the disc. This will mean either a new laser assembly or complete CD deck. Try the cleaning disc first and if that does not work take it to a service centre or just use it as a radio cassette in the garage.

The "no disc" messing indicates that the disc is not readable. This can be due to a dirty lens on the optics or defective optics. If you can gain access to the CD section, try cleaning off the lens of the optics assembly using a cotton swab and alcohol. Let me know if that does not resolve the problem.

Dan

Failure of this laser is quite common giving a no disc message in the HiFi's display. A defective laser can also be responsible for an intermittent no disc message and general playability problems with CD discs. Part number Optima 7 / 7B Laser Assembly

If your CD player dose not respond to normal commands, there is a problem with mechanism. Have serviced

If you used a cleaning disc, try again only this time open up the unit and clean the lens using a cotton swab and alcohol. Those cleaning discs rarely work. Let me know if yuo still have a problem.
Dan

Maybe something's jamming it, try fixing the drive, so it comes out.

Maybe you should fix the cds or clean them or try to fix the disc reader in the radio.

Have it first cleaned by a cd disc cleaner or if you are familiar with cleaning the lens manually im sure that will work...or try inserting an original cd's,if it works with that find the solution to the settings of your device...

A desperate longshot but the only i have to offer: It might be a problem with the electricity. if possible try and install it at another location in your house, using other electricity sockets. If the problem is solved by relocating the system, you might be looking at wiring problems (audio equipment can be very sensitive to that), if changing anything in the installation process solved the problem then the problem was probably due to bad grounding and "loose" electricity. Try making the installation as "clean" as possible, avoid tangled wires and stressing a single socket with too many electric appliances. If the problem persists, and you are still under warranty, I suggest contacting your local JVC dealer. If you're out of warranty I suggest locating a professional repair person. Good luck