Error: Failed to find a solution, Singular Matrix, Returned solution is not converged.-Feature: Stationary Solver 1(sol1/s1)."

Hi,

I have a laminar flow module and it is based on an imported mesh. I changed the java heap space according to the tutorial in this COMSOL forum and also changed the iterative solver to direct solver and it allowed me to run and solve my model. Everything was good one week ago and it could be solved and obtained reasonable result. But today when I opened this file and tried to run this one, it showed
" Failed to find a solution.
Singular matrix.
Returned solution is not converged.
-Feature: Stationary Solver 1(sol1/s1)."

My biggest confusion is how is it possible to appear an error when I reopened the same file? (I didn't make any modification!!!!!) Can anyone help me??????

Best,
Nan