The basic problem is that it takes energy to produce energy. Hydroelectric, steam and nuclear power plants all require energy to start up. Steam and nuclear plants require enough energy to bring a large amount of water to steam before operation can begin. Hydroelectric plants need power to open massive valves which opened manually normally take 300 turns.
Hydroelectric plants are the easiest to start from black-out conditions.
Other types of plants might take days to restart. Since catastrophic transformer failures are extremely infrequent, power plants do not keep spare units. While waiting for a new transformer, a power plant would be inoperable. Whenever an electrical device is started up, it requires an instantaneous surge of power that is greater than the normal operating power requirements for the device. For example, a flourescent light requires an instantaneous power surge 10 times greater than its normal operating requirements to start up.