I have experienced data loss for the first time yesterday in what sounds like a similar situation. I'm a long time Evernote user.
Yesterday my Mac (running Evernote 10.2.4) needed to be restarted due to some network issues I was experiencing. Upon restarting, I launched Evernote only to realize all notes created in the past week were completely missing. They were not available in the web client, nor on another sync'd device.
I had to manually recreate some important notes from memory, as I use Evernote for nearly everything.
Now I'm seeing the in-progress 'Syncing Changes...' update in the Evernote client, but I see none of the notes created yesterday have been uploaded.
As it looks, I'll need to manually export the notes I've created since yesterday as I can't seem to get Sync to work.
My support case opened yesterday appears to require 10-12 days turnaround time.
If anyone has found a fix or workaround (other than find an Evernote alternative) please do share.
-Adam