Microsoft-owned video calling app Skype is back following a 12-hour shutdown. Callers around the world, especially online workers who have reported glitches may now go back to work.

The service provider has spotted a network issue that prohibited users from logging in.  During the shutdown, users who are automatically signed in on the service see the app continuously loading. All contacts appear to be offline. Exchanges in the app cannot be accessed. A message says, "Skype is unavailable at the moment, check back later to see your news and alerts."

The problem was first reported at around 4 a.m. (EDT) of Sept. 21. On its own Heartbeat service, which updates the status of Skype connectivity, the company said: "Some of you may experience problems with Skype presence and may not see online. We have detected an issue with the status settings of Skype."

The company said that while voice and video calls are unavailable, users can still use the instant messaging part of the service and the outage doesn't affect Skype for Web which is still in beta.

In the age of global workplace collaboration, Skype has become a vital tool for English as second language teachers for many Asian countries as well as writers and other online workers.

ESL teachers in the Philippines who teach Korean, Japanese and Chinese students found it impossible to work during the outage and had to reschedule their classes. Writers and information technology professionals had to find other ways to reach their foreign colleagues.

One of the pioneers in video calling services, Skype has edged out its competitors with its 4.9 million daily users. The app was launched in 2003 and was purchased by Microsoft in 2011 for $8.5 billion. The calling tool has 500 million installs on Google Play.

The app is available in over 28 languages and is used in almost every country around the world. Thirty percent of the users use Skype for business purposes.