Hi Victoria,
Sounds like an Esri grid... You can read more on Esri Grid formats here: ArcGIS Help (10.2, 10.2.1, and 10.2.2)
Please note that a raster in Esri Grid format consists of various files and folders. There will normally be a folder with the name of the raster, which contains like 5-7 ADF files and you will have an info folder with arc*.* files.
If there are not complete it is possible that you don't have all the files required for ArcGIS to recognize it as a raster.
Kind regards, Xander