An open hot reading on your outlet tester means that the wire that should be 'hot', or connected to power, isn't getting any power. There could be a couple of reasons for this. If the outlet reading as open hot is connected to a switch, the switch could just be in the off position. However, if the outlet isn't connected to a switch or the switch is on, then the hot wire has disconnected somewhere. Having a disconnected or intermittently connected wire can be a fire hazard, and should be looked at by an electrician.
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