First of all I would suggest using the Lumia Software Recovery Tool (make sure you use the usb cabled supplied with the phone as that is a factory cable) and if that doesn't work, you will need to flash a compatible Amber ROM via Nokia Care Suite. However if NCS is too complicated for you, I would suggest using LSRT:
1) Go to C:\ProgramData\Microsoft\Packages\Products\ [Your Phones RM Number For example: RM-821]
2) If there is folder with your RM number, take a note of it and either delete files inside the folder or move them into another folder created into other directory i.e desktop.
3) Place the new files into a folder with your Phones RM Number {if there is no file and you are not sure what your phones RM number is or need to find the product code, just run the recovery tool and it will tell you on the left handside)
I presume you have already searched the 1520 forum?
If so perhaps try XDA to see if anyone posted a compatible amber rom.