From the homepage:

"Due to the discovery of a serious security issue a decision was made on Sunday 15 June 2014 to take down the website. The full extent of the issue is still being investigated but in an attempt to be as honest as possible to our members we must make you aware that we have reason to suspect that all or parts of the database may have compromised. Passwords at Project Euler are strongly encrypted using a one-way hash, but if you use the same password at other websites then it is strongly advised that you change it. We are extremely sorry for this inconvenience. At this time we can provide no more information and there is no indication when Project Euler will return."