Don't know your exact configuration, but my radiator has a pressurised header tank on top of the actual radiator and then an overflow tank which isn't pressurised. You will probably find the overflow tank is empty
The header tank has a level switch about 1/3 the way up and this senses low coolant. The header tank also has a normal radiator cap and you will need to remove it to add the coolant. For safety, don't remove the cap when the engine is hot.
Your setup may be quite different - eg you could have a pressurised header tank and no separate overflow tank but your manual should tell you how to top up coolant.
Since this has happened immediately after winter, you will need to check that your coolant does give adequate freeze protection and that the coolant loss isn't due to damage from freezing
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