Edward Zalta, the editor of SEP, shared the following:

The Stanford Encyclopedia of Philosophy is currently offline due to a power outage on campus at Stanford University. We expect to be back online shortly after power is restored.

Meanwhile, you are encouraged to make use of the SEP’s Amsterdam mirror site at: https://seop.illc.uva.nl and the Sydney mirror site at https://stanford.library.sydney.edu.au/, though the search engine at the Sydney mirror is currently offline due to a pending system upgrade.