All the colours normally displayed on a monitor screen are made up of combinations of the primary colours Red, Green and Blue. If you see only the Blue colour, it means the G and R components are missing.
There are three main points where the G and R signals can go missing : 1) in your graphics card, 2) at the monitor cable, and 3) inside the monitor.
To check if the graphics card is the culprit, test your computer with another monitor. If the display is normal, then the gfx card is OK.
Alternatively, you can test the monitor by connecting it to another computer.
To check the monitor cable, boot your computer and gently flex the cable, especially just behind the VGA connector that plugs into the CPU. See if the normal display comes back while you're flexing it. Some of the individual wires inside the cable can get broken if you've been using it with the cable pressed against a wall.
Repeat the process at the point where the cable enters the monitor.
This is not really the place to discuss details about exactly where inside the monitor those signals can get lost. That will involve technical details about the electronic circuitry which is not really appropriate for this forum.
But to give a general idea : If it's the CRT (picture tube), then it's not really worth repairing. If it's in some part of the drive electronics, then an electronics technician should be able to repair it.
Slap the side of the monitor and see if the display colour returns to normal, at least temporarily. If it does, then it could be a dry solder or loose connection inside the monitor.