Computer shuts down randomly

I have a Macbook Air. I have been experiencing various problems with it. First, I noticed a couple of time that my internet server, firefox, would close down. A pop up message informing me that there was an issue. Then I noticed that my entire computer shut down on its own, randomly and even when my computer is fully charged. weird. When I turn it back on, there is a note telling me It shut down due to a problem. That is it. I wonder if its doing this because I dont back up my computer or that its a couple years old now, or because I dont shut it down at night. Dont know. If anyone has an idea can you let me know why its doing this ?

Thank you