I feel you - the exact same thing has happened to me, both yesterday and today! Yesterday was particularly scary as I had logged in to do my numerical test and decided to procrastinate by doing the practice one, and the site went down half way through my practice one. This may be the only time that procrastination has saved me haha!
As for today, I managed to get in to do a verbal practice test but have now been sat waiting while the page loads for around 25 minutes. It was slow to load the practice one, not this slow mind you but it did load in the end! I assume it's related to high volume of traffic. Hang on in there!