The Ring of Negative Protection does not prevent Level Loss.
See this link for more info, but Negative Levels and Level Loss are different effects and an item that protect against one won't necessarily protect against the other.
A character who loses a level instantly loses one Hit Die. The
character’s base attack bonus, base saving throw bonuses, and special
class abilities are now reduced to the new, lower level. Likewise, the
character loses any ability score gain, skill ranks, and any feat
associated with the level (if applicable). If the exact ability score
or skill ranks increased from a level now lost is unknown (or the
player has forgotten), lose 1 point from the highest ability score or
ranks from the highest-ranked skills. If a familiar or companion
creature has abilities tied to a character who has lost a level, the
creature’s abilities are adjusted to fit the character’s new level.
With Level Loss you are literally going back one level. You lose ability score gains, feats and skill ranks. Your Base Attack Bonus, saving throws and class abilities are set to whatever your new level is. Your experience total also goes down to account for the change.
For all intents and purposes you no longer have a level X character but a level X-1 character and you have to recalculate accordingly.
Level Loss is permanent and the only way to 'undo' it is to gain more experience.
An affected opponent takes a –1 penalty on all skill checks and
ability checks, attack rolls, and saving throws, and loses 5 hit
points and one effective level or Hit Die (whenever level is used in a
die roll or calculation) for each negative level. A spellcaster loses
one spell slot of the highest level of spells she can cast and (if
applicable) one prepared spell of that level. If she has more than one
spell at her highest level, she chooses which she loses. A psionic
character loses access to one power per negative level from the
highest level of power he can manifest; he also loses a number of
power points equal to the cost of that power.
Negative Levels on the other hand have a very specific effect. Whilst you do lose an effective Hit Die you only take fixed penalties which aren't based on your Class.
Negative Levels can be healed and there is a chance that they will dissipate harmlessly after 24 hours with a successful Fortitude check. However if this check fails then the afflicted creature will permanently lose a level instead!
The Ring of Negative Protection will prevent the user from ever gaining negative levels.