Computer monitor says "No Input Signal" and goes to power save?

It was working perfectly, then when I switched on yesterday, that's all it said. I've checked all the leads - everything is connected as it should be, lights come on as normal, computer's humming happily to itself... It's not on warranty so I'm hoping I can fix it myself - can you suggest... show more It was working perfectly, then when I switched on yesterday, that's all it said. I've checked all the leads - everything is connected as it should be, lights come on as normal, computer's humming happily to itself... It's not on warranty so I'm hoping I can fix it myself - can you suggest anything please? Thanks!
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