Monitor Detecting Computer Input

[killler0000]

My Issue: My monitors aren't detecting any connection from two of my computers. Only a day or so ago they were displaying one of the computers perfectly fine. I am currently using one of my older computers on said monitors. My current theory is that something went wrong with the graphics cards in the affected computers. What I cant seem to figure out is why, even if the cards are removed, can't I get the display from attaching the cables directly to the motherboard. I just purchased one of the affected computers from a friend and it was working fine for him (I had used it previously just to check). The graphics cards I'm using on these computers are a gt 660 (?) and a gtx 710. I've reset everything, I've switched out cables, I've unplugged anything unnecessary, nothing seems to be working. Can anyone help me fix it or supply me with sufficient input on what parts I may need to switch out. I also don't have access to the specs because for the most part I didn't start with the computers and didn't receive all the information. 

[Mark.]

let's start at the basics and work up from there.

remove any add-on graphics card from one of the PC's.
and just connect one monitor for the time being.
if you get a signal on the monitor and can get into BIOS, check the display settings in there.
some motherboards only allow one monitor despite having multiple display outlets, however that was more with older boards, most boards for ages now, detect multiple monitors on the inbuilt ports.
other motherboards also disable the inbuilt ports if an add-on card is detected.

are old is all the gear?
and it was working until recently?

as to specs, thry this;
get Speccy from here; https://www.piriform.com/speccy/builds in Speccy, click File > Publish Snapshot > Copy to Clipboard > Close.
 now you can paste(Ctrl+V) that link into a post.