I re-installed windows 8 and now my laptop doesn't recognize my external monitor

Like the title says, I re-installed windows 8 and now my laptop doesn't recognize my external monitor. I tried, bu to update the drivers for my GPU, but it said they were up to date. The monitor worked fine before I re-installed Windows. If it is helpful, I have an AMD Radeon 7640G. And I'm connecting the monitor to it via VGA. Thanks. 

Right click on the desktop, click screen resolution, then click detect. see if your monitor is detected. if so, then you're good.

When I click detect nothing happens.