Hi,
Sorry for inconvenience. Your data should not be lost but your geometries are likely to be invalid.
What template are you using? How did you change the webmap, using ArcGIS.com viewer or the builder? Was there something specific to your data and basemap (different projection...). Are your picture stored in a Feature Service?
With more details I may be able to help more but from now your best hope is to try fixing your geometries using ArcGIS.com viewer by switching back to your previous basemap for example.