File Folder Monitoring
I assume you intend to build something on your own. Things to look into here are


launchd: Allows programs/scripts to be run based on certain conditions. Of special interest for you is probably the WatchPaths parameter which allows to watch a path/directory for changes and triggers a program/script if necessary. There are race conditions involved here, but for your case you probably can catch them easily. The full format for launchd configuration files is available with man launchd.plist
Shell/bash scripting: The code to execute if a file arrives probably isn't too complex, so some basic scripts should be enough
How to send an email from command line? 




On second thought, I would ignore the WatchPaths part completely. Just use launchd to run a shell script every 30 minutes (or whatever is appropriate) which, if no new file has arrived since when-ever, sends a notification to wherever.