What can you do?
The fingerboard on an instrument can come loose, either completely or only partially.
This can have various causes,

The fingerboard on the instrument can come loose, either completely or only partially.
This can have various causes, e.g. a fall of the instrument or very low humidity.

Can the fingerboard on the instrument be moved or does it rattle?
If the fingerboard has fallen off completely due to external influences, the first thing you should do is relax the strings.
The neck can warp very quickly if it is no longer stabilized by the fingerboard and all the string tension is on it.
Place a piece of cloth under the tailpiece and remove the bridge.
A fingerboard that has fallen off is not something you can fix yourself, but must be repaired by a luthier.

-The glue used for this is glutinous glue, not standard wood glue from the DIY store
-The gluing surfaces must be clean and fit perfectly
-Special clamps are used when gluing so that nothing slips
-The exact direction of the fingerboard can be determined during gluing