Sounds like something changed your display settings - there are many reasons that can happen without direct changes from you, though it usually doesn't.
To change it back, go to the Apple menu, and choose system preferences.
Click on displays, and then choose a resolution appropriate to your monitor - I can't know that without being there. A typical base monitor size is 1024 x 768. Or my MacBook runs at 1280 x 800 by default.
When you choose a new option, you will be asked to confirm the setting, or it will revert back automatically. If you choose an option and end up with a blank screen DON'T TOUCH ANY KEYS, and in 15 seconds, the screen will revert back to the old setting. That is a resolution your monitor won't support.
If it keeps happening, pay attention to what you are doing when the screen shrinks (ie are you closing or changing applications, and if so which ones?) PowerPoint presentations cause frequent problems in this regard if you don't first exit presentation mode before leaving PowerPoint, for example.