in arcmap, rightclick on the layer, bring up properties, then look for the projection and extent in the general/source tab. It is likely that the file had a defined projection, so changing it to something else, will foul it up. Check the extents, do they like they are in decimal degrees (-180 to 180 EW and -90 to 90 NS range) or are the numbers large (could be in one of hundreds of projections.)
Webmaps tend to use a web mercator projection (although not guaranteed), so you should check your environments and files before you change them
