Have u logged in once b4 and exited without giving the test??....if yes, then u've been marked as one who has completed the test, and u wont be able to give the test. But if not, then check the mail id with wich ur logging in, might be a mistake in that. Otherwise the site works pretty fine. Atleast i hvnt come across any errors. If all this doesnt work, 1st thing 2morrow mornin, do to the test page, and instead of logging in, go to the "LIVE Chat" link just above the instructions. There u can chat live, with a microsoft representative who will help u out with ur prob.....
And hey dude, don wrry.....u'll get to give ur test......Best of Luck!!!....