It seems the Music Publishers Association, a UK organisation with industry links to EMI, PRS and many of the other usual suspects, is being quite problematic for them.
A short extract from their forums:
The MPA, without notifying us, sent to our domain registrar GoDaddy a bogus DMCA takedown notice. GoDaddy took the entire IMSLP.ORG domain down. IMSLP has filed a DMCA counter notice with GoDaddy, however, the DMCA seems to require the registrar to wait no less than 10 days before restoring service. This means that IMSLP is inaccessible from IMSLP.ORG during this period of time. We will be working to restore service as soon as possible.
What is the MPA complaining about? Rachmaninoff's Bells, which is public domain both in Canada and the USA: http://petruccilibrary.org/wiki/The_Bells,_Op.35_%28Rachmaninoff,_Sergei%29
MPA's claim is entirely bogus.
Further legal clarification is available here on the IMSLP forums.
Basically I think this looks like bullying, plain and simple. It's scandalous that such a valuable resource should be taken offline because "traditional" music publishing companies have their knickers in a twist over something that isn't even illegal.
Workaround: You can still reach the site by using either petruccilibrary.org or petruccimusiclibrary.org. Note, however, that some links on the site that refer to IMSLP.ORG may be broken; you will have to manually replace IMSLP.ORG with one of the two above domain names manually in the URL bar.
Anyone who is interested in suing or helping to sue the MPA under DMCA section 512(f) (misrepresentations) please contact imslproject