Hi Charlie,
I would start by going through ArcGIS Server logs. There, you may encounter lost connection to SQL Database, in which case, it would be an IT Issue and/or a SQL Database Admin to further investigate.
This of course, is that when you mean add a layer, you actually mean, adding a Mapping Service or FeatureAccessService and that the service 'data' is empty. which would still show as 'layers' in the web map, but no data.
Id do a query through the mapping service (MapServer) directly to see if records are pulled.
e.g. arcgis/rest/services/[Folder]/[MapServerName]/MapServer/[TableIndex]/query
e.g. ObjectID > 0 GET