Google’s popular calendar app is up and running again following an outage yesterday that left it unavailable for up to four hours for some users.

Google first acknowledged the problem when it reported Calendar as being down on its Apps Status Dashboard at 9:47 a.m. ET.

According to reports, users across the world were receiving a “Server Error” message when trying to access Calendar. It’s not immediately clear how many countries were affected by the outage.

Google issued at update at 11:33 a.m., saying that: “Google Calendar service has already been restored for some users, and we expect a resolution for all users in the near future.”

Later, at 2:36 p.m., Google revealed that the problem had been fully resolved, adding that it would be investigating what caused the crash.

“We apologize for the inconvenience and thank you for your patience and continued support,” the company said. “Please rest assured that system reliability is a top priority at Google, and we are making continuous improvements to make our systems better.”

As of now, it’s not known what caused the problem, which reportedly spilled over to Google Hangouts, where some users reported having difficulty joining conference calls through Calendar invites. Many of the Google Web services are interconnected through Google Drive, so it’s likely the Hangouts issue was directly related to whatever disabled Calendar.

It’s not clear how many users were affected by the outage, but judging by the number of complaints on Twitter, the problems were certainly fairly widespread.

When #GoogleCalendar is still down 15 minutes before your first meeting and you don’t remember which office it’s at. pic.twitter.com/lscdY7U7uy

— Tony Hellenbrand (@TonyHellenbrand) June 30, 2016

Oh that’s cool, @googlecalendar is down two days before my wedding. Not like I needed any of those details or anything. Thanks, Google.

— Michael Hoffman (@michaelhoffman) June 30, 2016

This isn’t the first time Google has experienced a major service disruption, and it probably won’t be the last either, so those who’re dependent on Google’s Calendar app for organizing their daily lives would do well to have some kind of ‘Plan B’ in place should it ever happen again.

Main image credit: 422737 via pixabay.com