Your device is in Download Mode. I don't have that device, but if the battery doesn't come out, you will probably need to use ADB and send an adb reboot to get it out of Download mode. If you don't know how to use ADB, then you can just let the battery die. I don't like that option though because the same image will be on the screen for an extended period of time.
Does the screen look like this:

To use ADB:
- Download the USB Drivers from Samsung
- Download the Android SDK
- connect the device to your computer via USB cable
open a command prompt where the Android SDK was installed and change to the platform-tools directory. for example, my sdk is installed at c:\android-sdk so I run the following:
C:\android-sdk> cd platform-tools
then try to reboot with adb:
C:\android-sdk\platform-tools\> adb reboot
if that doesn't work try the following command:
C:\android-sdk\platform-tools\> fastboot reboot