Because you'd have to have engineered the whole thing for that purpose right from the get go. In theory you can run the generators in reverse and push water up the hill into the basin. In practice this may not work for a multitude of reasons (priming, encasement, rotation reversal, cavitation, impeller and impeller housing design).
Also missing in vast majority of the dams is a lower reservoir. Pumping up the water from a river/canal below the dam would result in a dry river bed just below the dam rather quickly
Spain does have few (probably 4 according to ESP Wikipedia) pumped-storage hydroelectricity plants. Supposedly these are being used nowadays to store excess of energy produced by fotovoltaic plants. No idea how fast these can switch from storing energy to producing it and if these were used to help during the blackout.
> No idea how fast these can switch from storing energy to producing it and if these were used to help during the blackout.
Typically ~10 seconds.
The bigger issue is if these have blackstart capability. (IE, if they can switch to generation when there is a blackout, or if they need power from the grid to start.)