If you get an UnreadableWalletException at startup the Bitsquare.wallet got corrupted.
That happens very rarely and luckily recovering is not too hard.
We use rolling backups, so at every start the wallet gets backed up with a timestamp up to 15 backups. The the oldest will be deleted.
So what you need to do in such a case is to look up for the last wallet which was ok.
First do a backup of the whole data directory, just in case.
You can find the data directory under those locations:
Mac OSX: /Users/username/Library/Application Support/Bitsquare
Windows 7,8: C:\Documents and Settings\username\AppData\Roaming\Bitsquare
Windows XP: C:\Documents and Settings\username\Application Data\Bitsquare
You can find the Bitsquare.wallet file in the data directory inside the mainnet/bitcoin folder.
Further inside there is a mainnet/bitcoin/backup/backups_Bitsquare_wallet/ folder and there you find all backups. Sort by date and use the one with the date when the wallet was still ok.
Copy that to the mainnet/bitcoin folder, rename it to Bitsquare.wallet and start Bitsquare again.
You can check if the balance is correct by opening the emergency wallet too with cmd+e.
There you can see the total balance. Compare that with the sum of available balance + reserved balance. It must be the same, otherwise report it here.
Please be reminded that doing regular backups, writing down the wallet seed words and using a password to protect your wallet is highly recommended.
So far no bitcoin has ever been lost with Bitsquare, but if you lose your wallet (e.g. due a HD crash) and have no backup we could not help.