This error is caused by the ESET Service. You need to start it, and luckily, it takes a few clicks. Go to Start > Run > Services.msc. Locate the ESET Service in the list, then right click it. Change its working mode to Automatic if it's Disabled/Manual, then manually Start the service. Apply all the changes, and the ESET application will work normally again. If you wish, a restart is also recommended.