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There are many, many possibilities here and you haven't provided much information.

You need to tell us more about what kind of PDE you are using, what kind of problem you are trying to solve, and what solver settings you are using, etc, as well as boundary conditiions, meshes, etc.

With that caveat, just as a quick guess, let me I suggest that you try a different preconditioner or a different solver. You'll find many options in the Solver Parameters dialog under the Solve menu item.

when I click equal sign on the menu bar to solve my problem, I get some messages on the log page

warning: ill conditioned preconditioner. Increase factor in the error estimate.....
Error: Failed to find a solution

please give me some suggestion as quickly as possible

An ugly tip: if you don't have enough time, great experience with solvers, and are dealing with a <300k DOF problem (=me), then go with direct solvers like PARDISO and save yourself some time. It will take longer probably, but easier to use and usually more user friendly.

I am a student of mechanical engg (M-tech) in NIT silchar, India. I am designing a microactuator in comsol multiphysics which is basically a cantilever consisting of three layers. Top layer is of 1 micrometer thick titanium layer, middle one is 2 micrometer thick silicon dioxide layer and bottom layer is of 3 micrometer thick polysilicon layer. When a voltage is applied across titanium layer, heat is produced because of joule heating effect and temperature of the cantilever rises. As different layers have different thermal expansion co-efficient, there is a deflection at the tip of free end. I am getting some deflection when applied voltage is less than 1.2 volt. But when is more than 1.2 volt, it doesn't yield any result. I dont under stand what is happening. I am doing it in 3D taking fine mesh from free mesh parameters.

All layers having rectangular cross-section and one end of the cantilever is fixed
Length and width of the cantilever are 200 micrometer and 20 micrometer respectively

The problem is basically consisting of- structural, heat transfer by conduction and conductive media dc