You will need to count the blinks.
Here is the Service manual
[link|https://www.dell.com/support/manuals/en-nz/latitude-15-3520-laptop/latitude_3520_cyborg_sm/system-diagnostic-lights?guid=guid-18007183-1088-472b-95aa-832516c4982f&lang=en-us|Blinking codes|new_window=true]
It may be a memory issue. I believe this model has two memory slots. Try taking out and re-inserting. If you have two memory modules take out one and try the remaining one in both slots. Do this with both. Some times just removing and re-inserting fixes the problem or one module is defective or rarely one of the memory slots itself is defective.