If they are yellow, and look thickened, then fungal nail infection is most likely. Unless it is one small patch, then one course of drugs alone is unlikely to get rid. It can take over a year if the infection is on multiple nails, and quite low down on nail bed.
My doc prescribed me a strong paint-on treatment, and prescribed me 3 boxes at a time to keep prescription charges down for me.
However, if they are just yellow looking, not thickened or unusual in other ways, it could just be that you wore nail varnish too long without taking it off, and your nails have discoloured. In which you need to let the nails be free from varnish for an extended period. Try giving yourself foot baths with lots of lemon juice squeezed in, that eases the discolouration.