Here's the specs -> http://cdgenp01.csd.toshiba.com/content/product/pdf_files/detailed_specs/satellite_L755D-S5363.pdf
Try removing (1) stick boot and check then swap with (1) stick and boot once again and check. Next with (1) stick try booting with (1) stick in each DIMM Slot. -- The first part checks the stick for a bad one, and the second test if you have a bad DIMM slot. Note the DIMM slot can be dirty. Therefore, using Isopropyl alcohol dampen the RAM's contacts and insert, clean, repeat, remove for 5~10 minutes to dry and try booting again.
Q - what EXACT set of (2x4GB) did you purchase? ; provide a link.
Also, open the Resource Monitor and see if >4000MB is in the HW Reserved; see below. If it is then the BIOS has turned-off one DIMM; typically a compatibly/bad stick/bad DIMM -- neither good. Chances are: 1. Bad stick, 2. Incompatible RAM. Compatible RAM -> http://www.crucial.com/store/listparts.aspx?model=Satellite%20L755D-S5363&Cat=RAM
; only buy in sets. The 'problem' is OEM BIOS doesn't allow you to change anything. You only want DDR3-1333 SO-DIMM RAM @ 1.50v or 1.35v.