You can try adjusting your field sizes and see if it happens to be scrunched up between two other fields where you can’t see it. If you hover over the edge of the field you see a black cross/arrows where you can then hold your left mouse key down and attempt to adjust field widths.  

You may be able to get it to show by simply expanding your fields or by dragging one of the other fields over to another spot and then refreshing your screen. To move a field simply hover over it and then drag it to where you want to place it between an existing field. The green arrow will appear and allow you to drop the field on the screen.

If that does not work, you will need to contact Orion Support and have them delete the record for your user ID in the object state file that contains the field placement settings for AR Manager.

Dragging a field and placing it on another spot on the grid is shown in the below picture: