My computer monitor lost its blue color screen and green is seen. Sometimes it becomes normal for few minutes. what/where could be the problem?

Did this just happen without the monitor being disconnected for any reason?

Check the monitor connector at the computer to ensure it's fully engaged and correctly orientated. (I've seen them forced on backwards.)

If the connections are alright, it's either the monitor or the video adapter going bad.

It's also possible the video card (if not onboard video) may have partially worked its way out of the slot through enclosure heat etc.

Can you attach the monitor to another computer to see if the problem is still there?