Had the same problem after Win10 upgrade, my 3520 had a black screen and 3 beeps.  NONE of the traditional resets/setup/fixes worked! The only way I got mine and a neighbors 3520 going again was to remove the CMOS battery, which resets it enough to let you in the BIOS via the "F2" key at the now visible -Dell start-up screen- and then you can set the BIOS to reload the default values and restart.  Now you should be able to get in to your previous windows OS or Win10 (depending on how far the Win10 Upgrade got before it died) -in my case back to Win7 and Win8.1 for my neighbors.  The hard part is getting to the CMOS battery... take the laptop apart (all the way) AC/battery removed, remove keyboard (4 small push in  tabs along top of keyboard) then remove 3 cables (all marked with blue color/tabs) 1.) flip brown cable connector clasp to the right to remove keyboard cable, 2.) lift brown connector to remove power cable (upper left) and 3.)  lift brown connector to remove track pad cable.  Next remove 2 screws from the hole now open from where the keyboard was removed (the only 2 screws down brown plastic holes, middle left and upper middle) Now flip over laptop and remove all screws from bottom and then flip back upright and separate top of case from bottom, you have to gently pry apart at several points (guitar pick is a great tool or small screw driver) around case until top pops off, set aside.  The CMOS battery is a round flat cr2025 type battery in a holder under where the track pad was, at the front middle of mother board (pops out by small screw driver inserted in the slot on the left of the holder). wait a few minuets for the charge to disapate (could be as much as 30 mins. wait if it is a new battery or newer laptop, mine only took a couiple of mins....) pop it back in the holder and then reassemble the laptop (reverse what we just did).  after putting the main battery back and plugging in the AC hit the power button and the Dell start-up screen should come up, press "F2" to get in to the BIOS and move over to the tab to restore factory defaults and save them.  now you can get back in Win (your version) and restore it back to what you had or fix/finish the Win 10 update.  I fixed the problem drivers/settings etc as others have around the net (just search for your model/hardware/problem) and follow those leads and now I have 2 working Win10's running on 3520's! I can get you pics if it would help...
I currently have an inspiron 15-3520 laptop that I bought in 2012, that came with Windows 8 pre-loaded. I have updated to Windows 8.1 and are wondering if I can get the free upgrade to Windows 10 when it comes out. Any response would be appreciated, thanks.
Windows 10 Upgrade - Dell Inspiron 15-3520