If the old monitor still exhibits the half-screen problem when attached to another computer, and another monitor works fine with your current video card, a likely explanation would be that you've had a failure in the vertical scanning circuitry inside the monitor.
DMR
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A monitor/TV repair shop can most likely repair it for you, but depending on the value of the monitor, the repair cost might not be worth it.
Before going that route though, I'd ut the defective monitor on a working computer first. If the monitor still exhibits the problem, then the fault does lie in the monitor.
DMR
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