HazPop and the SEDAC Hazards Mapper get some of their data from external sources. When there are problems with the external sources the data can not be displayed on the app. The Aerosol Optical Depth (AOD) data originates from the MODIS instrument on NASA's Terra satellite. Data from MODIS is currently degraded after after the instrument was brought back online following a satellite maneuver in February 2016. Imagery based on that degraded data is currently not being distributed. Once the instrument is brought back to proper operating temperature the data will again be released.
For more information on the status of Terra/MODIS see https://wiki.earthdata.nasa.gov/display/GIBS/2016/03/03/Terra+Spacecraft+and+Imagery+Status