You probably renamed the NeutronMissileWeapon. if you wanted to keep the name change you need the change the objects that call the weapon. In this case, modify the SUPERWEAPON_NeutronMissile in the ObjectCreationList.ini file and change the NeutronMissileWeapon to whatever you named it.
If you didn't want to change the name then it's probably some error with your syntax (e.g. Not ending weapons with "End", etc.) or you accidentally deleted the NeutronMissileWeapon, here's the code for it:
Code:
Weapon NeutronMissileWeapon
PrimaryDamage = 0 ; not used for this weapon (it's "special")
PrimaryDamageRadius = 0 ; not used for this weapon (it's "special")
AttackRange = 99999 ; not used for this weapon (it's "special")
DamageType = RADIATION ; not used for this weapon (it's "special")
DeathType = NORMAL
WeaponSpeed = 99999 ; not used for this weapon (it's "special")
ProjectileObject = NeutronMissile
ProjectileExhaust = NeutronMissileExhaust
FireFX = WeaponFX_NeutronMissile
ProjectileDetonationFX = None ; not used for this weapon (it's "special")
RadiusDamageAffects = ALLIES ENEMIES NEUTRALS
DelayBetweenShots = 0 ; not used for this weapon (it's "special")
ClipSize = 1 ; not used for this weapon (it's "special")
ClipReloadTime = 0 ; not used for this weapon (it's "special")
AutoReloadsClip = No ; not used for this weapon (it's "special")
End
Just copy & paste it into your Weapons.ini file if you don't have it (do a search first).