Q: I,ve lost my videos and imported photos when downloading the new iOS 8

Check, if this help text lets you find your photos -

See:  Get help finding your Photos in iOS 8 http://support.apple.com/kb/HT6464

Have you enabled the "Summarise Photos" option? Then disable it.

By default, Photos displays a representative subset of your photos when you view by year or by collection.

To see all your photos, go to Settings > Photos & Camera, then turn off Summarize Photos.

And did you migrate your photos from iPhoto?  Some photos may still be in the iPhoto Library, that is no longer supported:

See: Migrating from iPhoto for iOS to Photos on iOS 8