I selected by mistake to lock my laptop and now it won't do anything. Can't open, close or restart. All I have is a black screen. What can I do? Thanks.

When the laptop is locked, user gets prompted with a message saying that it is locked and only the person who locked it can unlock it with a password... but your case seems different, as if there's something wrong with the moniter or maybe the hard-disk...

For unlocking the laptop try this:
1. switch on your laptop and then hold-press the on/off for 10 seconds (till the laptop turns off by its self)
2. switch it on again

If this does not work, try this:
1. make sure the laptop is turned off
2. take out the battery and put in again
3. switch the laptop on

If it does not work try this:
1. Connect your laptop to the power cable
2. Turn it on

Best thing to do then, take it to a technician...

If you just wiggle the mouse, or tickle the mouse pad, or press any key, it should wake up the monitor from the power saving sleep mode.

Then you can just enter your password if you need one and resume your work.