Product key not working for Windows OS

Hi,
Recently I have reformatted my computer and I have installed the Operating system, using the Operating system disk which I have got during the time of purchase of my computer. I have the OS Key printed on the sticker, which is fixed on my Computer CPU.
When the installation prompts me to enter the serial key, I just enter the same which is in the Sticker. But the code was not accepted. It’s a computer purchased before three to four years, so I may not be able to call the tech support assistance since the warranty is over . Really looking for someone to help.
