Restart Hearthstone and HDT.
If the problem persists: See here on how to manually set up the log.config.
Problem still persists: Make sure the Hearthstone path is set correctly. options > tracker > settings > set hearthstone path (There were user reports of this not working, to manually do this: Close HDT, go to %AppData%/HearthstoneDeckTracker (Win+R, paste), open the config.xml with any texteditor and look for <HearthstoneDirectory>...</HearthstoneDirectory>. Set this to the correct path.The correct path is the one used in the Battle.net app options (Battle.net icon > Settings > Game Install/Update > Game Installation > Hearthstone)
Check if the log files are created at <HearthstoneDir>\Logs and written to. If there aren't any logs or if the logs are outdated:Unblock Hearthstone in your antivirus programsExit Hearthstone, delete %LOCALAPPDATA%\Blizzard\Hearthstone, start HDT and then start Hearthstone again
If you are using the same Hearthstone installation since beta, try reinstalling Hearthstone (see here)