If you have Constraints defined in the project and processing is failing (or producing strange results) you should first try the processing with Constraints turned off. This can be done in the second tab of the Processing dialog.
If the project still does not process with Constraints turned off then there is another problem (such as incorrect references or bad camera parameters). See the Processing Troubleshooting section.
If the project processes with Constraints turned off, you need to study the Constraints to see if there may be a conflict. Reduce the Constraints defined to just one or two by opening each Constraint in its Constraints Dialog and turning it off. Then try to process again with just these fewer Constraints active. By choosing which Constraints are active and reprocessing you can determine which Constraint or set of Constraints is causing the problem.
If you find one or more Constraints that will not process you should first look for incorrect definition (for example including a line in the Parallel Line Constraint when it is clearly not parallel to the other lines) or for conflict (for example including a line in two Perpendicular Line Constraints when those Constraints conflict).
A more difficult situation is when all the Constraints are defined correctly but there are too many of them on an object or some sub-set of objects. In this case we call the object "over-constrained". This can cause the optimizer to fail. In the case one must reduce the Constraints on the object until processing works.
Note: Constraints are not intended to correct gross errors in a project. For instance, if your model does not look very good because of misreferencing, incorrect camera parameters etc., then Constraints should not be used to correct for these deficiencies. First, ensure your model has basically the correct shape, then use Constraints if needed to refine the model.