If you have a solar standalone (solar panels only) system there’s nowhere to send excess electricity and so the system shuts down for safety until the grid is back up and running. If you have a transfer switch installed with your solar you will be able to use the electricity you produce through your panels (i.e. when the sun is out). If you have a battery storage system, you can continue to use stored energy to either a dedicated sub panel or the whole home depending on your system design.