If you leave month end wizard in the middle of month end it will lock you out, rather than leave month end, just open the area you want to look at. i.e An Owner, in a new tab.
Then you don't affect month end, but if for some reason you have left it and when you try and get back in, you get a pop up saying Month End is in session, then you need to reset the lock
Go to System Settings, Click on the drop down arrow next to More and choose "Reset Transaction Lock"
This will ask you if you are sure, say yes and then save and close
Then refresh your screen by the Internet button
Try Month end again and you should be back in.