You're building a PC. You install the CPU and RAM, and then you plug the motherboard into the power supply. You hit the power switch for the first time and . . . nothing appears on the monitor screen. Everything is turned on, the monitor is connected to the VGA port, the fans come on but there's no video.
You're not alone. I daresay everyone who builds from components has experienced this. In the "lingo", this is referred to as "the computer doesn't POST". When you're building a new system, getting it to POST is the first big milestone.
I was going to write my own treatise, but I found this much better one at Tom's Hardware:
"No POST", "system won't boot", and "no video output" checklist
The checklist is very thorough - just methodically go through and check off each bullet point. At the end there are some useful pointers on how to proceed when building a new system. If the problem doesn't go away, then you have to face the possibility that one or more components may be bad. It's handy to have spare components around, so you can swap them out.
By the way, POST is an acronym that stands for Power On Self Test.