Can you adjust and "OK"oro therwise accept the contrast and get back into the main menu?
If not, given that the device is ~16 years old, there is unlikely much you can do.
One would be forgiven for suggesting that you can do as much, if not more, with even a budget smartphone AND have the benefit of having contacts, diary settings backed up onto the web. Google mail (gmail) andits attendant calendar, Google Drive coud storage and other features are free (OK, you do "pay" by letting Google use your browsing habits to tailor ads for tat)
Try a Motorola G4 Play - with incentives you could pick one up new for under £80, delivered.