Depending on many things the tire may just have a slow leak and need re-inflation, I have an air compressor just for such occasons.
Of course it is not always that simple. On high pressure tires (Motor home for example) if the tire is FLAT (not just low) there is likely sidewall damage.
On low perssure tires,, So long as you don't drive on it re-inflation may be all that's needed. Once you move it, replacment is likely the only option.
I do have a story about other complications, but not today./
Home is where I park it!