Hi, I just started using unreal and was having this exact issue and found various posting about it without an answer.
1.) It was the right viewport.
2.) Grid was checked.
3.) Version 4.14.3
I tried changing the view of different viewports(using a 2x2 layout) to the right view and there was no grid there either. I tried unchecking and rechecking the grid option, that also didn’t work.
1.) I right clicked in an empty spot of the right view port with NOTHING selected, and select “Go here”. The view went blank(Could no longer see my geometry).
2.) Zooming out a ton until I can see the sky sphere, it appears the “Go here” command set the camera position very high on the Z axis.
3.) I zoomed back in all the way towards the center(Where my geometry was) and the grid was suddenly fixed.
EDIT: I lost the grid again(Repeating my above steps fixes it each time), but I may have found a clue. It seems to happen any time I use the Focus command. If I have linked orthographic views enabled, the bug will occur no matter which orthographic view I use Focus in. If I disable linked views, it only happens when I use Focus in the right view.
Additional Notes: It appears using “Go Here” on any orthographic view excluding Top always puts the camera into empty space(Off the grid). This is with NOTHING selected, it functions correctly in other views when I have something selected. If I use it with something selected in the Right view, the bug happens and I have to go through the 3 steps again in order to fix it. It also seems to sometimes just happen for no reason at all while zooming in and out with the mouse wheel, not sure if it’s connected to linked views or not.
Hope this helps
EDIT2: http://imgur.com/a/7v5Gt
Here’s an imgur album showing the process.