Check the Memory
***Always switch the PC off at the rear power switch and unplug the power cable, before un/installing internal components/cables ***
Clean both the memory RAM stick and the Dimm Slot the memory is installed into.
These steps are not usually because the memory or slot is dirty as such, but rather it is to make a good solid connection when reseating the components
1. Shut down the PC using the power button on the top of the PC, hold button for 15 seconds, or until the PC shuts down completely.
2. Turn off the PSU with the switch at the back of the PC. Remove power cable.
3. Remove both RAM sticks and clean gold connectors on both rams using alcohol or nail polish remover. And put them back. Make sure the ram makes a click sound when you push each corner in. A short video on how to uninstall and clean the ram: https://youtu.be/0GhxMyPgYOo
While the memory is out of the slot, clean the slot with compressed air, or a fine, clean brush, then blow down the slot to dislodge any dust or debris after brushing. Then reinsert the memory
4. Plug power cable back in & turn PSU back on.
5. Turn the PC back on using the Power button.
6. Test the system
If these steps do not resolve your issue please see the following article