Under normal circumstances, a thing is forgotten only by occupying the mind with something else, i.e. by losing interest in it through acquiring interest in something else. A thing cannot be forgotten by simply willing it to be gone. Loss of long term memory can result from damage to the brain, a faint original impression or agitation or excitement of a moment. A second type of forgetfulness is simply refusing to bring it to consciousness. For many a war or other tragic experience is dealt with this way.
If you're wondering if you left the electric iron plugged in, you did.