This is one of the most common issues you will find in the Mautic forums. And the problem is, if you are facing this there is not one but one of many reasons this could have happened. Like FrankS101 said it can be a permission issue.
I would recommend opening the log files inside the Mautc app directory and viewing the error logs generated by vim app/logs/mautic_prod-DATE.log
and checking out what the error is to use the solutions provided below.
You will have to set the owner of the entire Mautic directory to the owner of the running HTTP server instance, for example, Nginx service should have nginx or www-data as user group you have to check it out which one it is.
If you are using apache, check the write permissions of apache to the folder especially these two folders bundles
logs
.
You can also try increasing your PHP allocated memory inside the php.ini file. Do it wisely though it will depend on the amount of memory the server already has and how much you want to allocate to it.
You can try disabling some plugins that you might have installed recently that could be causing conflicts.
You can try clearing the cache and reload, this one can help too. remove the cache by rm -rf app/cache/
This can be the result of a faulty installation, and you might have to reinstall to update the database, so follow the installation procedures properly in order to get a bug free Mautic installation.