If a "The Phantom Knights" trap monster's effects are negated, is it turned back into a trap or no?
Depend on the exact Text but most Phantom Knight Monster has the condition: "(This card is NOT treated as a Trap Card.)"

If the trap has this text, it will not return to be trap if the effect is negate, since it is not an effect but a condition. It is also uneffect by every card that is applied to Trap Cards (MST, Royal Decree, etc) and keeps being a Monster if it change to face-down position.

Without the condition, the answer from simples_mathematics appeal

For Ruling look at the wikia:
http://yugioh.wikia.com/wiki/Card_Rulings:The_Phantom_Knights_of_Shadow_Veil

Also note, that OCG-Ruling don't official apply in tcg tournaments, where the judge has the last word, but still should be used as navigation points