KVM Switch-box not coming on

A while ago i purchased a KVM switch-box for my pc and my xbox. Basically it uses 2 VGA inputs (one from my PC, 1 from my xbox 360) and one VGA output, for my screen,
In the past this has worked perfectly, but all of a sudden it has ceased to function properly.
Because it uses power off the inputs, you have to have at least one of your inputs plugged in. Now, it only turns on when my xbox is on, but not just when my pc is on.
I know this can't be something wrong with the cable, as i have tried two cables, and that you can still switch to the pc and use it as long as the xbox is on.
My KVM Switch-Box

I don't know that unit, so I can only talk generally from my experience with my one. The 1st thing to check is that the cables are securely connected, more so with the keyboard\mouse cables, without these connected mine wont power on as it thinks nothing is connected, so this is a highly probable cause of your problem. Next is switches, mine has a button on the top to switch between which input is displayed, if yours has such a thing see if pressing that makes a difference, next visually inspect the cables, remove them and inspect the pins inside to make sure none have been bent over or anything. Now try a quick test, reverse the leads, what I mean by this is connect the leads from the PC to the Xbox, and vice versa, see if the problem remains, and if it does the problem should be reversed as well, by putting the known working (Xbox) leads to the PC, and the PC working fine then it is a fault in the other lead, if the fault stays on the same side as it currently is then it is either the PC not sending a signal down the lead to tell the KVM it is there, or the KVM switch itself is faulty. Should you be unsure if it's the switch or the motherboard sending the signal, ask a mate to try the KVM on they're system, that should conclusively prove the switch one way or the other. TBH with my knowledge about such things, there is nothing else that I can think of that could be behind this, so either something above will get it working again, or else you will find out where the fault lies, hope you get it sorted.

Thanks i fixed it, i had to plug the mouse & keyboard leads that are meant to go in the back of my pc, into the switch. Since it uses the old non usb plugs for mouse & keyboard, its useless to me. My suggestion it get a usb switch instead.

Yeah I thought that was the issue, it needs the ps2 keyboard\mouse connectors (keyboard at least) to pick up there is anything connected, glad you found out what you needed anyhow.