Hi @ceebeegeebee ,
You can check if the discs are spinning or not by inserting a disc and remembering what position the label was in and then allow it time to check the disc and then eject the disc and check the position of the label to see if it has moved in relation to what it was when it was first inserted.
Has it worked OK before when inserting the exact same discs?
''If it hasn’t,'' check that the discs are compatible with the player. (see p.8 of the [https://au.yamaha.com/files/download/other_assets/0/318600/CD-S300_RTAL.pdf|user manual])
''If it has'' then here’s the start of the [https://www.manualslib.com/manual/1160171/Yamaha-Cd-S300.html?page=13#manual|test mode] procedure from the service manual.
Once in test mode you will be able to test the functions of the CD mechanism.
The manual also has the schematics and a full parts which will help to find the problem that will hopefully be shown by the test mode procedure.
Hopefully a start.