That was fast. After updating their Macs to OS X 10.9.3, many users noticed that their /Users folder was gone. Actually, it was there, but somehow the OS update rendered the folder invisible. There were some 3rd party patches to fix it, but late yesterday Apple updated iTunes to version 11.2.1 and although Apple's description of the update doesn't mention the issue, details of the update on Apple's support pages makes it clear that iTunes was the culprit.
If your /Users folder was invisible (or even if it wasn't) apply the iTunes update and your folder will return.
The bug occurred on my MacBook Air, but didn't show up on my Mac Pro. Applying the update fixed it on my laptop, and no reboot was required. The update is recommended for all Macs. You can find it on the Mac App Store or under Software Update in your Apple Menu.