Why does my Lumia 520 keeps restarting after the Windows 10 preview build 10136?

I downloaded the update of preview build 10136, after the download, the installation started but the phone got switched off without the whole installation being completed. It automatically restarted and it keeps doing like that.

use wprt to go back to 8.1 then update again.

can u tell me what is wprst???

Windows phone recovery tool

can u tell where i can get it plz help out i hve problem of installing windows 10 while installing it struck at some point and showing sad smiley. and wen again i press power button it restart to install and again same problem occures

Let me google that for you

i installed wprt but it's unable to connect to my device (lumia 520) as it keeps restarting over and over again.
is format the only option left?