You can turn it off by holding the power button down for 4-5 seconds. If that does not work and it still stays on I would unplug it and carefully remove the battery.
You can then replace the battery, plug in the power cord and give it a bit of time for charging, then try starting up again.
If your computer will start but you cannot access the OS, you could start up from a Linux Live USB stick (I suggest Mint 17 32 bit) by creating one with
UNetbootin, then click on your computer drive to mount it and copy the data off the drive.