Shut down the PC, disconnect mains power cord & open the case.
Remove all the RAM module(s) & replace them fully & securely in their slots.
With case still open, reconnect the power cord & boot PC.
If symptoms persist, try only ONE module at a time in the the first slot (will be labelled DIMM0 / DIMM1 or similar).
If the above works, try 2 modules & so on. Each time remember to disconnect & reconnect the power cord as applicable.
Get back if none of the above works. G