The problem could be at Mendix side, because someone asked the same question look at the url. https://forum.mendix.com/link/questions/91543.Ronald gave as answer this url https://status.mendix.com/ where you can find the updated status of the server.
It could also be that it's just the Web Modeler fault. Try using it with Crome browser as it will be less buggy and keep refreshing the page as it's still a beta version. Be patient and it might work!
If you're sure, that the problem is not on your side. You could write a bug report to Mendix. Checkout the support.mendix.com page for more information.
I am doing this for a college project and the experience has been frustrating so far. Thank you for pointing out the support link, let me try that see.