You could make a VERY simple design with just 1 or 2 standard items and try if it works OK. If so, the programs runs fine. You could also try a design you made a few weeks ago and see if it still works OK.
If you can make a succesfull rendering of a very simple design, than you may inspect the design with the problems. Maybe one of the objects you used causes the problems. By deleting one object at a time you can find out which object it is.
Alternatively check the forum section that deals with 3D problems. Sometimes graphics cards or drivers may cause problems.