Check first that it is not a single disk stuck inside.
Try pressing repeatedly the eject button on the on dash player while at the same time you start the stereo.
If that does not work you can try disconnecting the car battery to reset the unit, and then attempt again the procedure above.
Remember that you may be asked stereo code after disconnecting battery, have it ready.
If still no luck, then the unit must be removed from the dash to be taken apart, so to remove the stuck disk.
In that case the repair may require replacement of CD drive unit or front panel. When disk does not exject repair done from a local technician may be expensive. Usually better replacing the whole stereo unit.
PS: Here the procedure above illustrated in detail. The procedure is described for home DVD players,
but it is the same on any optical unit, including in-dash stereo. Naturally , since the unit is
an in-car system, you do not have power cord, but you will need to
disconnect battery to reset the system: player system won't eject disk.