Flashing globe at start-up

Hi, three or four times in the past couple of weeks, when I have started up my Macbook, it has failed to start up and there has been a white screen with a flashing globe (planet Earth) in the middle of the screen. I had to force shut-down and restart. Each time the restart has been successful. I googled the flashing globe and found contradictory explanations etc etc. Is this something I should be worrying about? My Mac is getting a bit old now. Any advice would be much appreciated.

Sorry, I don't ever have time to contribute much to this forum (and I feel like my knowledge is very limited anyway) but I really appreciate the help from many knowledgable people on here who kindly give up their time to help others! Thank you.