I've seen a very similar issue to what you describe. And, like you, I really have no idea why the display settings got changed as I know I did not alter them. It was really frustrating when gaming and I couldn't get the resolution and refresh rate I wanted. See if this helps you. Go to Settings / Display, and scroll down to Advanced display settings. From there, click the link for Display adapter properties for Display 1. That will open a new window and at the bottom click List All Modes. Notice how this window is similar to the old Control Panel stuff from days past! You should be able to select the proper resolution and refresh rate from there. You can also click the Monitor tab and adjust the Screen refresh rate setting if necessary.
There is one other possibility that could be causing your problem, but your description is different that what I experienced. Again, for some unknown reason, whenever the display would go to power save mode, and then refresh once I was back at the keyboard, the screen resolution would default to something other than what I was originally using. The real downside to this was that my desktop icons kept getting rearranged. The answer there was obscure and took some poking around the registry and either changing or deleting keys. I don't believe what you are experiencing is that, so I will defer that obscure key hunt for another thread.
