Hello, I’m Nathan, and I’ll be assisting you today. My livelihood depends on your positive feedback, so your satisfaction is my highest priority.
Sorry to hear you're having trouble.
The likely issue here is that as this is a smaller/older set, it is a relatively low resolution model, far below what the computer would normally output.
if you hook the computer back up to a normal computer monitor, then go into the resolution settings (right click on desktop, then Screen Resolution), then set the hdmi/2nd monitor output to 1024x768, that should provide a signal that the sony is capable of accepting.
If you can't get it via hdmi, you can try the vga connection as well (though that won't provide any audio)