Drop Shadow Issue

I should add that if you'd like to separate the Drop Shadow layer style from the layer to modify it separately from the content on its own layer, then you can select the layer and go to Layer > Layer Style > Create Layer.

If you applied your drop shadow as a layer style, you can double-click to the right of the name of the layer or below the layer to reopen the Layer Style dialog. Click on Drop Shadow or to the right of the name (not the checkmark and not the +) to access the Drop Shadow options if they're not showing.