let's consider a DC power supply of 5V with for instance 1A of maximum output current (i.e. its maximum output power if 5W).
Let's connect a load R = 1 Ohm. Ideally a voltage source of 5V will give to it a current equal to 5V/1Ohm = 5A. But it is out of the maximum current of our power supply.
What does it happen at circuit level? I suppose that our power supply will provide a current of 1A, so the voltage drop on the load will be equal to 1V. Where are the others 4V?