Looks like you're not the only one who has this problem.
Their twiter are full with people who has the same problem as you
https://twitter.com/BittrexExchange/status/988587634606866432For now, there are no words from Bittrex itself, you can either wait until the issue are fixed or you can contact them
https://support.bittrex.com/hc/en-us/articles/360001318332-Contact-UsI was lucky that I was using Google 2FA, it didn't effect my ability to login at all, maybe you should try to enable Google 2FA too in case something like this happened again in the future