Try doing a search of your entire HDD for the .dll in case it accidentally was moved. Check your recycle bin for the .dll
if you find it then it should be placed in the bin directory of your ArcGIS install location.
if you used defaults it should be
C:\Program Files (x86)\ArcGIS\Desktop10.5\bin
or whatever your current version is.
In Control Panel, Go to the Add/Remove programs section and try a 'Repair'
