i don't know your age. or more importantly have any way of seeing your toes and examining you but it may be onychomycosis -- a nail fungus. see a podiatrist or your primary care medical provider.
what causes very thick nails on 3 of my toes. index and middle on left foot and index on right foot. nail is very thick and also looks like it could be pulled up from from the nail bed where it connects at the cuticle of the toe. no pain or anything like that - just terrible to look at and i'm sure there has to be something wrong.