when you 've lost admin rights

I couldn't access my cdrom drives and it had something to do with the user rights. A short while later I could access my drives but had lost admin rights!!

The quickest solution I found and tried was:
Reboot, when entering the grub menu press c to prevent further booting. Choose: Ubuntu recovery mode. Then you get a menu: choose work in terminal
and enter: usermod -a -G admin username

After that's done (without feedback) type reboot.
Other useful commands
To get info on your cdrom drives: dmesg | grep CD-ROM
b.t.w. the command that got me into trouble was:
first: usermod -G cdrom username
and then:
chmod 660 /dev/(name drive)
found with
ls -la / media
or
ls -la /dev/cdrom*

Change username into your own username.