Nina - not an pc expert, but if the power light is on, it may have been placed into 'sleep' mode. You might want to take a look at your monitor settings through the Control Panel.. wish I could be more of a help...:)
Yikes.. OK, me no computer wiz.. BUT the 2 things to check for starters would be the cable connection from the monitor to the computer. Make sure its got a good connection.
The other thing to check of is:
Go to control panel, then display. In display hit settings. You should see a gray box with blue squares marked 1 2 etc depending on how many monitors you have. You have your default monitor(primary) and then your extended monitor.
Make sure the "extend my windows desktop onto this monitor" is checked.
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