The vast majority of electrical generation equipment, which is to say virtually all of it, is designed to either shut down or isolate from the grid when a disru

The vast majority of electrical generation equipment, which is to say virtually all of it, is designed to either shut down or isolate from the grid when a disruption of electrical grid frequency is detected. This is to protect both the equipment and the grid, in the case of mechanical generators and even some solid-state generators connection to a significantly off-frequency grid (e.g. due to severe load imbalance due to loss of interconnection) will cause physical damage to the generators and possibly to transmission equipment. So yes, windmills will come to a halt because they are designed to. Or, more specifically, they will feather their blades and continue to rotate idly while not producing power in order to ensure adequate hub lubrication. There is a significant concern that, even in a scenario where all power plants are undamaged, it would take a significant period of time for plant operators and distribution system operators to restore service due to the need to more or less restart everything. Something very similar happened during the California power crisis and the great Northeast blackout, in which undamaged power plants were offline for an extended period of time due to the difficulty of restoring grid condition to normal. Even under normal conditions of a stable grid, when power plants start (e.g. due to expected increase in demand) it takes hours from startup to connection to the grid. Most renewable generation methods (except hydro) and the newest natural-gas turbine plants can start much more quickly (e.g. 15 minutes), but this assumes that they will be connected to a grid which is operating at their design frequency, which would be very difficult to achieve if there has been a significant loss of generation. And just to worry a bit more, a significant amount of electrical generation, especially in plants capable of load following (quick response to demand), is via natural gas. Natural gas distribution also relies heavily on compressor stations which run on, well, electricity. These stations have backup power capability but recent experience in the Southwest, during the 2011 winter, shows that this is not entirely reliable, and that severe events can disrupt the natural gas supply as well.