There are numerous posts on this problem. None of the suggestions that I was willing to try worked for me.
But I deleted some recently installed apps from my iPhone and then was able to successfully sync.
Long story. I had always synced automatically when I plugged my iPhone (now 6S) into my iMac. I got the "iPhone could not be synced because the iPhone was disconnected" problem after returning from a week long trip. I usually sync every day or two. I didn't want to start over or even Restore as I was afraid of losing data or having to re-add lots of passwords. I must have tried twenty times to sync. Reboot, shut down iPhone, turned off everything on the iMac that wasn't needed. etc. Tried Wifi sync.
The sync would often fail after as long as hours of appearing to sync. My thinking was that something was corrupted, the kind of thing on a Mac you'd delete the preferences file for. And I tried to thing of what might have changed on the trip. I had installed a few new apps. So I deleted about six apps that I thought I might have installed while on the trip. And voila, sync worked. Seems like it still takes longer than it used to, but it's working. I have since reinstalled at least three of them and syncing still works.
Hope this helps someone. YMMV.