Symbian is a mobile operating system, and one of the first to be developed specifically to power smartphones.
The platform was created in 1998 by Symbian Ltd, formed as a partnership between several phone makers including Nokia, Motorola and Ericsson, plus Psion.
The software itself was developed from Psion's EPOC platfom seen in its handheld computers.
Symbian is most closely associated with Nokia, as the S60 software used in many of its phones was based on Symbian.
In recent years, Symbian was converted to an open-source project, but has gradually been dropped by every major handset maker other than Nokia in favour of platforms such as Android.
In February 2011, Nokia announced it was switching to Microsoft's Windows Phone 7 as its primary smartphone platform, although the majority of handsets it makes are still based on Symbian.
The issue affects only a small number of users, according to the company
Microsoft initially designed the new OS for foldables, but has changed the focus to single screen devices
Users had less than 60 seconds to save their work before the reboot
The company says the redesign will signal that "Windows is BACK"
Models up to and including iPhone 11 seem to be affected