Hi @sebastianwaloch
I couldn't find a service manual for the CTK-3000, but since the CTK-3000 and the CTK-2000 share the same [https://support.casio.com/pdf/008/CTK2000_3000_e.pdf|user guide], perhaps there's not much difference between them. Hopefully not.
Here's the [https://www.synthxl.com/wp-content/uploads/2020/04/Casio-CTK-2000-Service-Manual.pdf|CTK-2000 service manual] that may be worth a look
Go to p.13 (15 of 27 pdf) and run the Diagnostic tests and check if you hear sound at all.
If none heard then perhaps check if the fuse FU3 (see p.20) on the Main PCB M800-MDA1is OK