Unfortunately, it will be the Laser lens / head that has probably failed as they don't last forever, it happened to my CD 192. the good news is they are not expensive to replace, I had the technician at my local HiFi shop replace mine (because I'm too lazy to do it myself) and was pleasantly surprised by how little it cost to fit and configure.
If you are confident and competent you can probably replace it yourself as I believe these units are calibrated at the factory, so you don't need to adjust any settings in the new Laser, just replace and play.
I've just doing some quick googling; I believe part number KS213CCM is the laser sled and drive mechanism for the CD72 or Part number KSS-213C for just the Laser Lens assy. The original CD72 service manual calls up part number CDM14 for the CD transport, but I understand this is the entire Tray, Sled and Laser assy. There are also two variations on the CDM14 depending if your machine id the CD72 or the Latter CD72T (for reading CD text).
You should check with Arcam before purchasing against either of these part numbers though, maybe another more experience tech might chime in on this post and confirm.
Hope this helps.
LPSpinner.