It doesn't actually exit the program when you close it, so if you try reopening it nothing seems to happen. Check your tray on the bottom right (if you're using Win10) there should be a little JRNL icon, you might have to press the ^ arrow icon to find it.
Hope this helps!
For anyone having a problem opening the program after closing it the first time, this is the answer. You can either leave it in the tray, or change the "CloseToTray" variable in the config if you don't like this functionality. Took me a while going through the comments to find this and a bit longer to find out how to stop it from going to the tray.