1. Right-click on Lock button -> select 'Restart Screensaver Daemon'
2. Check if the xscreensaver daemon is running. At a terminal window, type:
pgrep -u your_username xscreensaver -l
where your_username is substituted with your username.
3. If it is running, then you can lock your screen.
If you still cannot lock your screen, try typing:
4. If after Step 2, xscreensaver is not running, type the following at a terminal window:
xscreensaver -nosplash &
Try locking your screen