First, go to where the daemon stores the blockchain. On unix based systems it will be in ~/.dashcoin. I am not running Windows, but if you are running windows, press windows key+r then type %appdata% and press enter. Go into the folder "Roaming". In there there should be a folder called dashcoin.
rename the dashcoin folder to anything you want other than its original name. This is so you can recover it if you ever need to.
Close any running simplewallets. Now open the dashcoin daemon. It will say it needs to sync, let it do that. It will take a while.
When it is done syncing, open simplewallet, open your wallet, and type "reset" and press enter. Wait for it to refresh.
The problem should now solved.
wallet.dat can be in %appdata% or in folder wher it is simplewallet and dashcoind?
Can you upload fresh blockchain? It need time to do sync.... so by that time coin will be worthless
Dont wory you will have chance again on bittrex...