How do I reset my phone after a failed update?

[Windows Central Question]

I got an update which I downloaded and installed. After restarting a screen with cog wheels came up and said that the update was installing. At about 67% the installation failed and the screen displayed a frowning emoticon. Since then,whenever I start my phone a screen displaying cog wheels comes up and stays for 5 seconds and then everything goes blank.This has been happening since two days and I cant get my phone to start. What could be the probable reasons for this?

 

[Teo Bartulovic]

You have to roll back to 8.1 with help of Windows phone recovery tool...