The only thing I can think of is if your email account is compromised.

The hacker could have deleted the Bittrex confirmation emails, have you checked the trash? (Pretty sure there's a delete forever button).

This is far fetched... but what could have happened...
Your laptop is compromised (either you used a public wifi not your home one or there is some dodgy spyware on your machine.
The hacker then could clone your Bittrex cookies (if you didn't log out) Thus duping Bittrex to thinking you are already logged in on the new machine (no need for 2FA).
The hacker then deleted the confirmation emails from your email account using the same cookie method above.

All of the above would be a lot of effort for 800$ but its definitely possible.

If I were you, I would:
1) Completely reformat your laptop.
2) Then login to your 2FA auth accounts and change the password to random generated ones, use KeePass or something to generate and store these.
3) Disable 2FA on all of your accounts (why? Wait for the next part)
4) Then wipe your phone.
5) Re-Enable 2FA on your accounts with your newly built phone firmware.

If you follow the steps above you should be good. (Also if you used a public wifi... don't do it again).

~Lucas