I'm making some assumptions here so please be kind:

Likely you where on an old fork of the NEOS chain and your transaction never took place on the "live" blockchain.    Have you tried loading a old backup of wallet.dat into a new NEOS client?  I believe the conversion to the "new" blockchain will be automatic. (because Syntax/twoturtles is a pro)  Remember: coin amount was reduced by 3:1 when we switched to the POS/Masternode code base v2. So you might be "missing" some coins.



Id start the recovery process for your Bittrex account ASAP. Ive logged into accounts and 'found' 1000$ of dollars of crypto Ive forgotten about and was worth pennies last time I logged in.  Good luck and happy Neos !