Depending on which flour you feed your sourdough with and how long ago you last fed it, it can develop quite a different smell. Sourdough fed with light wheat flour usually smells very mild and flowery. Sourdoughs made from wholemeal flour are usually already somewhat spicier in smell and taste. Especially sourdough based on wholemeal rye can sometimes develop a somewhat sour, acidic smell. However, this is not a cause for concern, but a normal part of fermentation.
If the sourdough is not fed for a longer period of time, the yeasts start to starve and produce alcohol. Then your starter can have a small "alcohol smell" or even smell quite chemically like nail polish remover (acetone). If this is the case, you can simply pour off the liquid carefully and then feed your starter again. As soon as a fusel layer forms, it is definitely time to feed quickly so that the microorganisms do not die.
However, if your sourdough smells foul like rotten eggs, then this is unfortunately a sign of faulty colonisation and a reason to dispose of it.
It is best to sniff your sourdough when feeding it so that you can recognise its characteristic smell.