Usually a blue cross marks where the electrical connector is for the internal lighting. It is for the maintenance people to press when installing the panel to ensure the connection is made.
Whilst flight deck crew may find pressing here will restore lost lighting, this is not what you are supposed to do. I have seen people pushing, thumping, banging and then breaking things in an attempt to restore lost lighting or a failed instrument.
Write it up and let the maintenance people fix it.
Happy flying.
P.S. I work both sides of the fence.