It's not just you. MyIdentifiers.com, the site in charge of ISBN registrations and sales in the US, has been down for the past week. What was originally planned to be a one-day maintenance period has stretched on for a week, with no end date in sight.
Bowker has not explained why the site remains down, merely telling visitors that the site "is currently undergoing an extended maintenance window".
I do however have a report from an author who told me their rarely-used credit card was stolen after they used it buy ISBNs from MyIdentifiers.com, so perhaps Bowker found more security issues than they were expecting.
Update: It has been confirmed. The site is down because it was hacked, and credit card numbers were being stolen
P.S. You can still submit data for a previously purchased ISBN (this is handled through another portal.) You can even buy a block of ISBNs - you just have to fax the order in.