Mapping Center Answer:

Are you perhaps using using lat/long for your X, Y coordinates and either meters or feet for your Z coordinates. If so, you actually have a valid TIN but ArcScene is assuming that the X, Y and Z units are the same. So what you get is a very, very tall TIN (which appears as a vertical line). The solution is to either:

1. Reproject the TIN into a coordinate system so X, Y and Z units match (e.g., UTM with the Z units in meters or State Place with the Z units in feet); or
2. In ArcScene, click View > Scene Properties > Vertical Exaggeration (area) > click "Calculate from Extent".

Solution 1 is a better GIS design. Solution 2 will at least allow you to display the data immediately.