what happened to the elo rating?

Server restart probably.

Rank is computed as percentage of active players.

You can check active players at: http://zero-k.info/Ladders/Full , now it says "Page of 40 (1968 records)", so it considers there are 1968 active players.

Some weeks ago, the number of active players was ~5000.

Examples that follow are approximations to illustrate the point: if to be certain rank you have to be in 5% of 5000 this means you must be in the first 5000 * 5% = 250, if on the other hand there are 2000 active players you must be in the first 100. If your position is 150, than some weeks ago you would have the rank (150 < 250), and now you would not have it anymore (as 150 > 100)

I am not sure why the server restart has such large impact on "Active players" (I asked, did not get an answer). I did not play in 1 month and seems I am still active.

Now, for your case, if some weeks ago there were more "inactive" players lower in the ladder than you than above, when server "re-checked" active player you can imagine your ladder position stayed more or less unchanged, but in percentage out of the number of active player it changed a lot.

One more thing that might look "strange": as far as I remember an actual rank change (up/down) happens only after you play a game and (win/loose). So currently there might be people that should be "lower rank" (because of explanations above) that still remain the previous rank, just because they did not yet play a game (or did not loose yet a game). But as soon as they do the rank will adjust. This happens is less common (most people are well within a rank), but thought to mention it, as it could be frustrating.

Rank isn't directly related to mmr, it's your position relative to the rest of the playerbase. The server reset didn't change anyone's mmr* but since there's fewer active players (and a disproportionate amount of the inactives are lower on the ladder) the same mmr is now in a lower percentile of active players compared to before.

*any ffa games which were being counted to casual mmr before aren't anymore after the server reset so this isn't quite true for people who had played ffas.

quote: I think if rather than happening at each server restart the "Active players" would be computed daily, things would appear much more normal.

As far as I know, it is supposed to be. But it does seem that it is not happening.