If the problem is underconstrained the solution is not uniquely defined and FEBio will not be able to find a solution. The most common example of an undercontrained problem is one where the rigid body modes are not constrained. A rigid body mode is a mode of deformation that does alter the stress in the body. Affine translation and rotation of the entire mesh are two ways to introduce a rigid body mode. In a (quasi-)static finite element analysis all rigid body modes must be properly constrained in order to find a unique solution. This can often very easily be achieved by constraining and edge or face of the model from moving at all.

If the problem is over-constrained, FEBio will usually find an answer but it is probably not the solution that was sought. The most common cause of an over-constrained model is one that has conflicting boundary conditions. For example, a force is applied to a node that is fixed.