|Don't panic just yet.|
Make sure all your connections are firmly seated and nothing got dislodged.
Sometimes very slight movement to the RAM sticks can cause them to not make contact due to tarnishing. Take them out and clean their edge connectors with a pencil eraser, then pop them in and out a few times to clear oxide off the sockets themselves.
This can apply to any circuit card edge connectors if disturbed.
Always pop back and let us know the outcome - thanks