Hi Apoorva Paranjape,
The mendix free app container goes to sleep after a while. Sometimes it just takes a few minutes to wake it up so that it can be used as usual again. I experienced wake-up times of up to 5 minutes.
If you are using an AfterStartup Microflow, check if it takes a lot of time. If so, this might be the reason why the app startup process is not pulling through.
Also check the mendix status – maybe there is maintenance being done for the free app containers.
For me this occurs once in a week(Using Menidx 7.x.x)
But it resolved its own, after sometime.