Why are ArcGIS 10.2 Color Ramps missing?

first make sure that your esri style is enabled.

1. go to layer properties
2. right click on any symbol and choose
3. properties for all symbols.
4. select the style references...
5. ensure that at least one of the style references is enabled.
   

You can manage the esri installed and your own style files from the style manager.
Go to customize > style manager.

you can drag and drop from any of the esri installed style (just enable it and it will be listed here) into your profile style (see [localdrive]/user/myname/appdata/)

If you are on a Win7 Computer go to:

C:\Users\your username\AppData\Roaming\ESRI\Desktop10.2\ArcMap\Templates

and either delete or rename the normal.mxt

ArcMap will take a bit longer to open next time (has to recreate that mxt) but the color ramps and styles will reappear.