Unfortunately to access your computer it needs to be turned on. To do this you will need to use a feature like "Wake On Lan".
WOL allows you to turn off your computer and wake it up by using a "magic packet" over the Internet. The "magic packet" is unique to your computer. Unfortunately you will need to do a bit of research to get this working. Have a look through this link for detailed help:
http://www.depicus.com/wake-on-lan/woli.aspxThis is all dependant on your computer having this feature. Most modern computers do. Usually you need to get into the BIOS (check manual) and turn this feature on.
If your computer doesnt have this feature, you may have "wake on modem." This allows your computer turn on when the phone line is activated. You can get freeware software to turn off your computer after a set time like 10 mins (for false alarms) and you can access your computer during this time and stop the "turn off" feature.
Hope this helps.