the only solution i could of think of is if you can access your home screen the go to the search bar and type in MSCONFIG after performing this go across to BOOT section and go into advanced options inside this you should find a slot that says maximum memory you want to set that to maximum or 8192Mb for you 8Gb of ram then save and exit and it should reboot the system and have all of your 8Gb of ram addressed.
if the pc is to slow to boot then start the PC in safe mode with networking and it will reduce the amount of ram that windows is trying to draw upon start up and then just follow as i previously mentioned.
Ash