Do you have multi WAN or VPN and use mangle roules to direct traffic certain way? If so, you’re hit by unexpected change in priority of marked packets. See e.g. this thread: http://forum.mikrotik.com/t/routing-priority-issue/158587/1
Hopefully you didn’t disable WinBox connection to MAC address, so use that to get back, and they you’ll need to change mangle rules to exclude local destinations (dst-address-type=!local).