You can open the machine and visual check to see if anything fell out of place.  Especially, reseat the memory module(s).

If the issue is persisted, it's best to contact seller unless you are knowingly purchased 'as is' unit. 

How can I reset the memory module?

Reseat, not reset.  Meaning, remove the memory module(s) and re-install it.  If you do not know how, just leave it be, as you could damage something during the process.  Contact seller instead.