Monitor and cpu not working with each other but working with other.

Hello,
Sir, after a very long period i normally turned on my computer but it didn't start. I couldn't understand what happened to it because when i am pressing the power button of my cpu it is working well but my monitor is showing each time that monitor is going to sleep. I could understand the problem so i took my monitor to repairing shop but they told me that monitor is ok. After this i took my cpu and vga cable but unfortunately it was also working. Sir my monitor and cpu is working with other cpu and monitor respectively but not working together. The repairer said me that ny cpu needs a graphic card worth 1400 to work monitor otherwise it will not work. Kindly help me to fix this problem. And also suggest me should i buy graphic card or is there any other way to get rid from this issue. Please help me.