I'm guessing here as I've never had a Cinq.

There are probably two fuse boxes.  One in the car in/around the lower
dash area probably on the driver's side.  The other will be in the
engine compartment not to far from the battery.  Also the battery
positive terminal may have fuses on its connection plate.  These fuses
are main fuses and will only ever blow if there are real serious
electrical problems which should have already been protected by the
normal circuit fuses. You may also have a few "Maxi" fuses.  These are
the very large blade fuse about 2cm in length.  These are also normally
protected by other fuses however some will be the sole fuse and are used
for really heavy current circuits like the radiator cooling fan, cabin
heater fan etc. and common central feeds.

With no circuit diags to look at the fact that the headlights/central
locking/windows have all stopped suggests a Maxi fuse or power
distribution connector have gone open circuit.

Nick /////