It happened to me...I found the way round it was to Google Purestorm, then get into the site via one of the options ( I think I used 'forums' ) then navigated to my profile then 'book marked' that link.
Didn't have any problems on my ipad, so I think my computer 'cached' the plesk page when it was defaulting to that and I can't clear it...I tried PS on my ipad after the plesk episode.
We did have a brief outage the day which could account for you seeing the Plesk page at that time but that shouldn't be the case now. Another user had an issue with this in Internet Explorer but when using Google Chrome it loaded no problem, I've just tested it in all browsers, IE, Chrome, Firefox & Safari and all loading the homepage as it should.
The first thing which springs to mind is that the browser has stored a cached image of the Plesk page and hasn't refreshed, this is easy to test by forcing the browser to refresh using the following keyboard combinations:
Windows: ctrl + F5
Mac/Apple: Apple + R or command + R
Please try this and let me know if it's still happening, thanks for the heads up and help troubleshooting the issue.