Emails are typically "On my Mac" or server based.
POP is always On my Mac.
Go to Finder
Go menu -> Go to Folder
~/Library
Search for Mail and Mail Downloads folder, and click once on each, and select Get Info from the file menu.
If the size is in the several hundred MB or GB the folders are at least intact for On My Mac.
POP based emailed email accounts where the size is only a few MB mean you may have logged in a different account on the Mac, or you have corruption on your Mac that needs to be addressed to find out why the email folder is so small.
Otherwise, IMAP and Exchange (Microsoft) accounts are stored somewhere remotely.
For remote email, if you are checking your email on an iPad, iPhone, or their web service like gmail.com you'll be able to see if the inbox is at least on the remote site. If they are there, and not on your Mac email client, it is possible the internet connection on the Mac wasn't set right or it lost an internet connection. WiFi less than 5 feet from the WiFi station has overlapping waves, and in another room over 150 feet away, may lose connection, especially with file cabinets or pipes in the way. Sometimes you can set your phone as a hotspot and try to check email using it as a WiFi hotspot.
We'd have to know who your email host is to find out more why your email isn't checking, and which style you are using to help you further.