Today is deadline day for getting in your 2006/07 self assessment tax return and paying any money you owe to HMRC. Submitting a self assessment tax return can be a big enough headache on any day, but those who've left it until today face an extra problem: the self assessment website has crashed.The site went down mid-morning today and at the time of writing, anyone trying to access the site just gets a rather understated message saying: "We are sorry but the service is currently unavailable" and "we apologise for any inconvenience this may cause."If you fail to meet the midnight deadline and owe more than £100 in tax, then you face an automatic £100 fine. Surely HMRC will have to extend the deadline in light of the system failure?Chas Roy-Chowdhury, head of tax at the Association of Chartered Certified Accountants thinks so. He told the BBC: "If the Revenue's IT systems have crashed, then they need to extend the deadline. They need to be pragmatic."Update: HMRC has extended the deadline to midnight tomorrow, Friday 1 February. Let's hope its website doesn't fall over again!