Go to home.mendix.com and check your app logs to see if there is any useful information about what is causing this issue.
If not, file a ticket at support.mendix.com
The Mendix free app container goes to sleep after awhile. It takes a minute or two to wake up the environment when you navigate to it when its asleep.
Heres some documentation that talks about it
If it takes longer than a couple minutes than I would definitely go with Mike's approach in filing a ticket