Hi, am sara,my computer shutsdown automatically, when I reboot it a siren kind sound comes from cpu and the BIOS display shows the system shutdown due to thermal heating. I checked the working of fan, its fine. I do not know what to do. How do I rectify it.

If you are knowledgeable at computers you could go out and buy Thermal Paste, which goes on the CPU right below the main motherboard fan

The first and cheapest step would be to open it up and clean out any dust clumps you can see.
Make sure the fan on the cpu ( should in near the middle of your motherboard) is cleaned as well.

Now try to use it.
If it still does this have someone you know that handles computers apply a new layer of thermal paste to your cpu.

The first step should solve your problem if its full of dust.