I agree with Nick's prescriptions. In my experience sleep mode (and hibernate) usually works well on laptops but often poorly or not at all on desktops. 'Budget' desktops often have 'on board' graphics and the drivers for the graphics part don't handle sleeping well, or not at all. It may not be Windows fault, though the update (if it was the big one - the "fall update") may have replaced the driver with the latest one Microsoft have. The manufacturer may provide Microsoft with a newer driver - or may not.

If it doesn't fix itself after a month or so you could try just updating the driver - the procedure is here. Easy enough. But I've done that for a video card and got 'you already have the latest...'. I had to get a driver from the manufacturer's website. I've never tried rolling back a driver - i.e. going back to the previous one.