User ID (not data) deleted suddenly?

Hi,

When waking up my Mac this morning, it suddenly seems he forgot my user name 'mg'.

My data is still there, but in the system logs for example I see:

Jan 18 10:08:50 mgmacbook xpchelper[90045]: getpwuid_r() failed for UID: 501, ret: 0, errno: 0
Jan 18 10:08:50 mgmacbook com.apple.launchd[1] (com.apple.xpchelper.F5010000-0000-0000-0000-000000000000[90045]): Exited with code: 1

The first relevant thing after the wake up in the log might be:

Jan 18 09:43:57 mgmacbook coreservicesd[57]: _scserver_ServerCheckin: client uid validation failure; getpwuid(501) == NULL

Also in the terminal, my prompt is:

I have no name!@mgmacbook ~ $

'whoami' command says:

501

instead of my user name.

In ls -l /Users I can see my directory as:

drwxr-xr-x+ 101 501    mg   3.4K Jan 17 19:33 mg

So the group information is still there.

I am a bit scared now of everything and afraid of trying a reboot as that might lock me out permanently.