Once you have located the fuse box, open it and remove the cover. The fuse for the heater is labelled "Heater" or "HTR" and should be rated at 15 amps. If the fuse is blown, replace it with a new fuse of the same amperage.
Please note that the exact location and labelling of fuses may vary depending on the specific model and year of your Vauxhall Corsa. If you are unable to find the heater fuse, please refer to your vehicle owner's manual or seek help from a qualified mechanic.