Can you verify if the last debug session is well terminated (Window > Open perspective > Debug). If no debug session is displayed, check if “openocd.exe” is still running in background and terminate it on your Windows task manager.
You also need to check if your board com port is not already busy (communicating with another software, such as ST-Link Utility) and the driver is not out-dated. (Nucleo boards has an embedded ST-Link/V2-1).
Hope it can help you.