Friends not able to join server.

I made a new vanilla server and my friends were able to join. But then i went afk for a while and when i got back my server was still on, there was no errors on the console. but my friend said he got disconnected and he can't join back. I portfowarded correctly. Idk whats wrong. Any suggestions? Idk if these have anything to do with this problem but when i went afk i also turned on my second computer and opened minecraft there to, and when i tried joining the server it says cannot reach server. So i then went to check on the computer that was running the server and then my friend told me the news.

 

Too many people don't know how to run a local server. Just rent a node.

 

I do know how to run a server. I use to run a public server but then i stop running it because it made my computer laggy. But it was a successful server with people that joined. But i don't know why its not working now. And if i don't know how to run a server how were my friends able to join earlier? I asking for suggestions

 

Check the power saving settings.
The hard drive may be turning off after 30 minutes.

I also agree with the DuneRacoon, servers belong on a server, not on a home computer.