I've had problems with this as well. Try changing the hoop you have set, then go into the preferences and set the grid. Then you can change your hoop size back and it seems to work for that session. I have to set my grid lines every time I go into Studio, sigh.....
The grid lines don't show in the 3D and 1:1 options, only in Normal, Image and Vectors. Are you perhaps using one of the 3D tabs?
You can get them back by going to the edit menu, then selecting preferences, then grid. Set them as you want them. Mine just disappear now and then but thanks to Mops I know where to find them!