I have no idea how you would get a dam to collapse with only a laptop and a network connection. As for the power plant, the operators would have to be blind and deaf to let a plant get destroyed.
The real threat is a cascading power grid failure due to undersupply, e.g. coordinated forced plant shutdowns. A few days without electricity at a large scale means reduced availability of medical and emergency services, no running water, failing refrigeration, no stoves/ovens for cooking for most of the population, no working gas pumps, no electronic payment, no banking (no way to get cash) etc.
The real threat is a cascading power grid failure due to undersupply, e.g. coordinated forced plant shutdowns. A few days without electricity at a large scale means reduced availability of medical and emergency services, no running water, failing refrigeration, no stoves/ovens for cooking for most of the population, no working gas pumps, no electronic payment, no banking (no way to get cash) etc.