Depending on the size of the puncture and whereabouts it is on your tyre will determine whether the flat tyre is repairable. If the puncture is limited to centre of your tyre’s tread and is less than 6mm in size, it should be repairable.
If the puncture is larger than 6mm, or is located on or near the sidewall, you will need to replace the tyre. The sidewall carries most of the load of your car, so any damage will compromise the integrity of the rubber and lead to a greater risk of tyre blowout.