Check this post on the Dell forum:
http://en.community.dell.com/support-forums/laptop/f/3518/p/19647188/20809088#20809088
According to this user, upgrading the BIOS fixed the problem and they were able to keep Windows 10 installed. You have to remove the CMOS battery in order to get it boot however, so there is some disassembly needed. You basically remove the laptop battery and all of the screws from the bottom. Then remove the keyboard and then the keyboard bezel. You can then remove the CMOS battery and let it sit for a while. I also disconnected all power and data connections from the motherboard. Here is a youtube video showing disassembly:
https://www.youtube.com/watch?v=LxgbChFc19I
I also found this service manual, which has disassembly instructions:
http://www.manualdevice.com/dell/inspiron-15-3520-service-manual
After it sits for a while (I let it sit overnight), put it back together and power it on. It took it a good 4-5 minutes to show the Dell screen on the one I was working on, so be patient. Once booted, you can either revert back to Windows 8, or you can try and keep Windows 10 by installing the A10 Bios for the Inspiron 3520 as described in the above post. My customer chose to keep Windows 8, but I also updated the BIOS to A10 for good measure.
Windows 10 will still try to install after reverting back to Windows 8, but you can disable the upgrade from trying to install. This site has good instructions:
http://winsupersite.com/windows-10/prevent-windows-10-upgrade-installing-after-making-your-reservation
So far, so good. I hope this helps.