What is it called when something is unimportant for several years, and then it becomes important?

Dormant

... 2 : marked by a suspension of activity ...

The idea lay dormant.

Other possibilities are latent or quiescent.

The latent potential went unnoticed. The idea lay quiescent for some time.

I don't often distinguish a phrase as having the perfect context but I think the common phrase ahead of its time, is perfect here.

A phrase I've been hearing more of recently is dark horse, defined by MW as

1a. a usually little known contender (as a racehorse) that makes an unexpectedly good showing

But little known could also mean that the dark horse is a brand-new thing; dark horse may not capture the period of being virtually unknown or overlooked. For this, consider sleeper, defined by MW to mean

4: someone or something unpromising or unnoticed that suddenly attains prominence or value: the low-budget film became the summer's sleeper

This term is commonly used for cinema as WP observes in its description of sleeper hit:

A sleeper hit is a term used in the entertainment industry for a film that plays successfully for a long period and becomes a big success, despite having relatively little promotion or lacking a successful opening. It is also used in a similar sense for music releases.

Consider the phrase "out of the shadows into the limelight."

There seems to be an unexplainable fear for A&M to step out of the shadows into the limelight of major college sports.

If you want a phrase that is synonymous with sudden fame and popularity after years of relative obscurity, consider the following:

• Catapulted into the limelight

His brilliant performance in the play catapulted him to stardom _{Dictionary.com}

Often used when referring to actors, film directors etc. whose talents were either ignored, unappreciated or underestimated by critics and the public alike until they get "discovered" in a hugely successful film which launches them, figuratively speaking, to overnight stardom. This is an effective analogy for the math concept "permanent".

Until recently the role of "permanent" had been gravely underestimated when it was catapulted into the limelight ....

Continuing with the analogy we have the expressions

• Rise from obscurity to stardom
  _{Google books}
• Rise from obscurity to fame
  _{Google books}

Permanent: The rise from obscurity to stardom
Permanent ignored by many in the world of mathematics until it finally rose from obscurity to fame