Why is OpenStreetMaps in Civi 4.7.24 rendering a featureless grey map?
As Graham has noted in a comment, the change in map reflects a change in terms of use and distribution methods in July 2016 that necessitated the removal of direct links to MapQuest tiles. 


MapQuest Issue - Direct Tile Access No Longer Supported?
mapquest direct tile access discontinued (SO)


The plain map you are seeing is the Carto Light basemap from Carto, which was adopted as of 4.7.19.

On our sites, we do not use the built-in maps for the public, but a Leaflet implementation for Drupal Views. Thus, this change only affected internal users who use the mapping, which does not occur often in our use cases. But for those who do, you can override this choice by adding a custom OpenStreetMaps.tpl template, with the URLs appropriate to your tile provider of choice.

 

I am not sure if any Carto tiles other than the basic light and dark (with or without labels) are licensed for this kind of use, however. The OSM wiki does provide a list of tile servers where you can explore alternatives.