I would contact a tax professional in your jurisdiction for assistance. They may contact the relevant tax authorities to negotiate an mutually acceptable solution to the lost records. This should serve as an important reminder to never rely on third party providers to store financial information that is needed for tax purposes. Instead one should store electronic financial records on computers under one's control, running Free Libre Open Source Software (such as GNU/Linux), with multiple redundant backups and in a format that is readable by Free Libre Open Source Software.
By the way I expect a fair amount of grief among former MTGox clients over lost / inaccessible records as the various tax deadlines around the world approach April 30th (Canada), April 15th (United States) etc.
Off topic, but isn't it funny you have to say "Free Libre Open Source" ? To me this is because of a case of political term obfuscation (a.k.a. people changing the meaning of words to hide important information from the masses) Can't say Free Software, can't say Open Source, Libre might catch on but right now you can't really use that either, so we have Free Libre Open Source Software haha
On Topic : I don't think anyone has a problem with MT GOX showing users their trade history, not only for tax reasons but also for personal records. At this point it is clear however than Gox is in "emergency mode." I believe that they realized (they should have realized sooner though, lol) that by leaving the website online they were doing irreparable damage to themselves and their customers. My feeling is that you will be able to get your information eventually (keyword EVENTUALLY). Either they'll try and help their customers or be forced to. There are enough people with enough money in Gox that I think things will become more transparent, not because Karpeles'n'co want them to, but because people are going to take matters into their own hands. (Look at the source code and database leak....)