Just a day? “I wouldn’t worry about that much.” You will get those messages from Googlebot time to time.
What you should worry about is if your site experiences frequent outages, despite length, because you never know when your site could be trafficked do you? That would be incredible information to know.
It’s poor user experience to find a site down. Sucks for everyone and their lost opportunity. If you get messages from Googlebot or Webmaster Tools frequently you may want to sign up for Pingdom or Yottaa and check your downtimes. Maybe it’s something you can fix or maybe it’s time for a new web hosting provider.
Of course sometimes it’s funny when the web site is down.