Your question implies that the tech level never falls.
WHERE THE COMMUNICATION IS CUT OFF MATTERS
The disaster that cuts off communication can be on Earth, not on Planet X. In fact, I would make that so, because if the disaster happens on Planet X's end, there will be impetus to rebuild those lines of communication. Should it happen on Planet X, you'll have to have a reason for them not to rebuild. On Earth, it might not be a priority-- but for the colonists, it would be--this is where they are from, and for Earthers, it might be a big resource sink when they have enough problems to deal with at home.
It might not even be any kind of disaster on either end, but rather that one of the relays in space needed to communicate had a malfunction or was hit by an asteroid or space debris--and it's too far for either planet to replace or repair.
ALL IT TAKES IS ONE WORLDWIDE REGIME
Lots of answers on here talk about how you'd need a Dark Age in which knowledge is lost, but that's not actually necessary.
Things that happened 2,000 years ago become more lore than fact. They might know their ancestors came from elsewhere, but might not know much about Earth.
Also, lots of this depends on societal structure. Were these people more concerned with building their own history and leaving earth behind? Could there be an age in history where this was true? After all, they were abandoned by Earth. Just one hundred year period of "book burning" could reduce the likelihood of Earth knowledge.
If people are angry about Earth "abandoning" them, that is, no longer communicating, the people on Planet X may, in a generation, want to destroy all remnants of Earth. The regime might see Earth as the source of all societal ills, and want to build a new utopia, which they may believe can only be done by erasing all notions of the Earth that they came from.
Furthermore, because this society could have been founded WITH the idea that they would be building a better world, so there could already be a faction which will eagerly use the communication cut off as an excuse not to look back. There may be some that reverently keep evidence of Earth-That-Was, but the majority can forget.