Brewology

You should be able to click on the home button, or the reset loader option from the app's menu.
If the app doesn't have those options just unplug and replug.

It won't hurt your wii, its just a hard reset.

Sometimes just pressing the power button once isn't enough, you have to hold it down. A lot of the newer homebrew has home button-functionality, but the older does not. If there is no way to go back to the menu, just hold down the power button on your Wii.