It is really hard to say, where your photos may have gone to.
When did you see the missing photos the last time? And what happened since then? Did you update any software? Which version of MacOS X is running on your Mac? Are you using iCloud Photos on more than one device and did you delete Photos on another device? Did you change your Apple ID?
Are you running out of cloud storage?
I take it, you already checked your Recently Deleted album?
Have you checked, if you are having more than one Photos Library? As a test launch Photos while holding down the Options key ⌥. Keep holding down ⌥ until the library chooser dialog appears. Now you should be seeing a list of all libraries that are available on your Mac. Try switching between these libraries to see, if another library will have your missing photos.