RAM problem after removing the battery

Hello everyone. Please tell me what could be the problem. Today I disassembled the computer, cleaned it and inadvertently pulled out the battery, after turning on the computer does not see all 8 gigs of RAM. That is, 8 gigs are installed, in the properties it sees 6 gigs and uses only 4 gigs. Will reinstalling Windows help? or how you can restore it. Thank you for your help !!!! Win 8.1 64, motherboard gigabytes ga-990fxa-ud3 latest BIOS version