The blanking plate would suggest there has been a card in there which is no longer in there.
First steps Al - unplug the power cable from the rear of the pc. Undo the side cover to expose the innards - it should just be two screws. Once it's open, take a minute to see what's inside, it should look like this.
Don't be daunted by it, note that there are various components that are plugged in like lego to the motherboard - touch the ground to earth yourself and check that these are seated correctly by pressing down on them gently and while you're at it, check that the cables connecting components, especially between the hard drive whcih is the size of a small paperback and the motherboard is sound.
Don't listen to the smart arses - not everyone knows about pc's but there are those who think everyone should!