Fluid flow and heat transfer in phase change material

hi
I'm using Comsol 3.4 ,I am solving a conduction- convection heat transfer problem and fluid flow
I got this error
Error:
Time 0:
Repeated error test failures. May have reached a singularity.
Last time step is not converged.

Please suggest me how to solve the above problem or give me some guidelines so that I can solve the problem.