A pen that has gone unused for a long time will as a rule fail to start because dried ink has blocked the feed and the slit of the nib. Before any treatment, establish what the body is made of. On models in plastic or precious resin, as they have been since the 1960s, patient soaking is usually enough: unscrew the section with nib and feed and stand it nib-down in a glass of lukewarm water for several hours, or overnight where the crust is heavy, keeping the filling mechanism out of the water. Afterwards rinse with clean water until no more colour comes away. With piston fillers you can instead draw water in and expel it again several times over. An ultrasonic bath speeds the cleaning up, but should be used only briefly and without heating.
Older celluloid bodies call for caution. Celluloid does not take prolonged soaking: it can lose colour and gloss and warp, and the cork seal of an old piston filler suffers equally. Such pens should be cleaned only briefly and, if possible, dismantled, though better still by a workshop specialising in old writing instruments. If the nib stays dry after thorough cleaning, or ink seeps out at the section, the feed or the seal is damaged and nothing short of a proper repair will do. With an inherited pen carrying a dedication engraving that is almost always worth having done, whatever the bare collector's value.