My computer is saying "Check Signal Cable" when I start it up.

I have powercycled the computer and monitor. I have tried 3 different monitors. When the computer comes on, all lights and fans come on as normal but it does not appear to be able to connect to the monitor. (All connections look OK!! )

It does not have a video card to reset. I don't know much about computers.. can anyone help me with this problem?

(I would very much appreciate it!! )