How can I lock my screen? When I click on the lock button, nothing happens.

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, the you can lock your screen.
If you still cannot lock your screen, try typing:

xscreensaver-command -lock

4. If after Step 2, xscreensaver is not running, type the following at a terminal window:

xscreensaver -nosplash &

Try locking your screen