CD-R's suddenly "unreadable".

My Levinson No. 39 has been doing the same thing, and since it's under warranty, I'm letting the factory figure it out...should be back in a week or two. Maybe they will tell me why it was happening.

I have commented several times about the instability of CDR's, "see vaporware". These are not archival mediums! Depending on the storage, age and plain deterioration of the CDR, it could be disc's going bad! IMO, this is a serious concern that should outrage every consumer, but it appears nobody cares about throwing their money away on something that will last maybe 1-3 years.

.......and you thought cassette were bad.

Make sure your disc can be read on other machines before you blame the playback deck.

News Flash: The RIAA has developed and secretly deployed new Satellite Technology capable of emitting low level radiation that effectively wipes out all CD-R's - when asked about this the RIAA said that recent laws passed by Congress allow the RIAA to take "whatever steps necessary" to protect copyrighted material. When pressed about the long term effects of the radiation and the recent reports of chickens being born with four legs - the RIAA commented "Not only has the RIAA effectively delt with copy protection, but we have also doubled the productivity of poultry farmers everywhere."

In a related story - http://digitalmass.boston.com/news/2002/05/20/copy_proof_cds.html

You should probably clean the laser. The pits are not as deep on CDR's and therefore are harder to read. My Sony is not supposed to play CDRW's but it does but it occasionally has to be cleaned in order for it to read them.

This is an elusive problem because it can be a lowered output from an aging laser, misalignment, tray movement problems or the fact you are burning the CDRs at 16Xs or 24Xs which seems to present problems with some players. I would first have a tech go through the player and establish output, alignment and all before blaming the CDRs.
Good Luck.