This error happens when the Application Pool that is associated with the web application doesn't start on Shared Honsting Server. This is caused by a number of reasons.
Your application may have crashed.
Your application may be crashing very frequently. If an app pool crashes for 5 times in 5 minutes, your application pool is disabled by IIS and you will end up getting this message.
Your site could be requesting for resources beyond the limit that has bee set on the Application Pool and therefore causing problem number 1 or 2 as mentioned.
If your website requires more resources than what is given you would need to let us know the resources that your website needs to that we can advise if it is possible to get them on Shared Hosting or not. If not, you will be able to run those resources on a Cloud or Physical server.
Because you are on Shared Hosting, you don't get Administrator priviledges, to resolve this you would need to contact us urgently so that we can resolve the issue for you. You can contact us from email@example.com or send an email to firstname.lastname@example.org. Most of the time this is quick to resolve.