My daughter accidentally turned on the Guided access, and the dilemma here is that we forgot the password, and we can't turn off the iPad to reboot, because it is stuck in app which she is using. How can I remove the Guided Access ?
That is a dilemma. If you know the password to access the iPad itself, hold the home and power button down until the iPad reboots.
When it starts, it should not boot back into Guided Access and you shouldn't have to remember the password.