The MySQL Connection could not be established.

[Naoyuki]

 Hello, first time having to do this so not sure what information you need exactly. When trying to upgrade my SMF (Softaculous) from version 2.0.7 to 2.0.9 I get the error "The MySQL Connection could not be established." When trying to log onto my site, I get an error code 503 on my PC, and "Sorry, SMF was unable to connect to the database. Please try again later." on my phone.
  Any help would be greatly appreciated, thank you.

[Kindred]

You should not upgrade with softaculous... Upgrade using the internal, smf patching option.

[Naoyuki]

Is there a way to get a file that was lost back? I notice there is a file missing when I check the hosting website or my FTP. So basically when accidentally misplacing or deleting a file from a server, where does it go? I think my issue would be solved if I could find that file.

[Kindred]

You can upload a completely fresh set of files using the large upgrade archive.  See the FAQ on loading a fresh set of files