computer wont start when ata cable is plugged to motherboard

Hi first of all i would like to thank you guys for paying attention to this post and i really hope i can find a solution.....


May i star saying that my pc was working properly i know is crucial to have details about pc but i dont have another computer at home and im at a c@fe right now so unfotunately im not able to do so i think my issue should be a simple question for you guys......

Pc was working properly then for some mistake i decided to take a look at my ram card's i took the 3 of them out cleaned them from dust then put them back thats all i did at that point,then computer just wouldnt start i connect the power but it doest boot,i UNplugged the hard drive cable(i only have 1 hard drive and 1 cd rom) then plugged it back but same issue.......i tried to do some troubleshooting trying to find out what the error or problem was,i finally have comed to the conclusion of what the problem is.......
when ata cable is plugged to hard drive and to motherboard nothing happens just get the green light on the motherboard meaning power is ok,but if hard drive is UNplugged to motherboard through ata cable i mean if is only connected to the power cable that conencts motherboard with hard disk it starts making that noise that is working but nothing displaying on monitor since ata cable is not connected......if i connect ata cable again to motherboard nothing happens and is the same thing over and over,ive tried the same thing around 50 times and always same response,if ata cable is connected to motherboard harddrive wont start UNLESS i unplugged the ata cable from motherboard harddrive will start but cant see anything on screen......thats my problem and i really thank you for reading this and i hope you can assist me to get a solution....thanks