From their homepage:
Dear members of PianoSheets.Org,
You've probably all noticed by now that this site is about to close down, due to the fact that the SysOp/Owner has decided to pull the plug of the tracker (and most likely the website anytime soon).
Since no-one except Hanzo (the SysOp) can access the actual site, there is no way to organize a decent rescue-plan apart from moving onwards to existing torrent-sites. There have been made quit a few suggestions in the forum. Here's a shortlist:

myanonamouse.net (MAM) - Predominantly an audiobook & ebook tracker. There's a 'Musicology' section with more than 12000 alive torrents
audionews.org - An audio software & samples tracker. There's a 'Learning' section with appr. 1200 torrents. Many of these are 'Music Learning'.
demonoid.ph - A general purpose tracker. There are plenty of 'Music Learning' torrents but you'll have to learn how to find them (no section).
what.cd - An audio tracker. There's a sheet-music section with plenty torrents. Not many new torrents
waffles.fm - An audio tracker. There are about 2000 torrents in the sheet-music section and quite a few musio learning videos in the 'E-Learning' section.
bitme.org - An e-learning tracker. There's a 'Music Learning' section with appr. 300 torrents.

There was an offer from anonbro to make his site a new home. Have a look in the Invites forum.
Most active members are already member of MAM, and since this tracker also has by far the most sheet-music available, it get's my vote as the best alternative for Pianosheets.
This will be my last post as Administrator of this site unless something dramatic will happen anytime soon. I'll still be around in the forum and on IRC.
Cheers, Robocat (former administrator)