Have you tried restarting the phone? (turning it off and back on) That may well be all you need to do to resolve the issue.
Should that not do it, then try going into "Settings" and then "Apps". Locate that app in the list and the choose to clear its "Cache". NOTE: Doing this will remove data and history from the app, so if there is something you wish to keep in it, then either save it somewhere else, or don't clear the "Cache".
Lastly, are you sure you have not been blocked by the service? I would check that as well.