The bass strings in a classical set are nylon wrapped with a thin metal winding. If the core is steel then that would be very bad for the instrument.
However, if the person at the store put on a true classical set then I doubt the strings alone caused the bending. You can look at the ends of the strings and if you see some type of soft fibers coming out it is a true classical guitar string, if you see a metal core then remove the strings and go to another store for help.
Classical guitars do not have truss rods so they cannot be "fixed" with a simple tweak. Your guitar could have warped from changed in humidity if it was not properly stored.
To straighten a classical neck you would need to go to a luthier who specializes in classical instruments. They may soak the neck, or expose it to extreme humidity to soften it, and clamp it between flat pieces of wood for a while. It could take weeks or longer depending on the severity of the warp.