Here's one solution I found, which does sound like possible reason you lost all your stuff (but you didn't say you were on any other computer):
This has happened to me when I accidently synced my iPhone on a different user account. It generates a 2nd iPod_Control Folder, pushing the first one as other data, and making all the music and videos as other data. AFAIK it's impossible to fix without jailbreaking or restoring.
The other plausible explanation is you just messed up your sync before you went on vacation. For example, if you added a ringtone and synced that, it would wipe your music, if you did an auto-sync. However, you would think you'd see that happen immediately and I believe you said you listened to your music while on vacation.
The other solution was the reboot, which I already told you about and you did.
The first solution doesn't help you however, since you don't have anything on your hard drive, so you can't resync. The lesson to learn is always have backups. Sorry.
As far as settings for syncing go:
Change your sync setting from auto to manual. You want to be in control of your sync.
Then you can just drag music to your iPhone as well as ringtones.
Make sure you go into the info tab and set up everything you can to sync (notes, contacts, calendar, etc.)
Back up, back up, back up your music and other important stuff. Backup your iTunes folder.
If I'm leaving anything out, hopefully someone will add to this.