Network Service Account password
Solution 1:
You should be able to change it back by going in to Service Properties and changing "Log on as" to "NETWORK SERVICE" (use "Browse" and it should populate the password fields for you).
Solution 2:
To change it back enter for the field This account: NT AUTHORITY\NetworkService,
then clear the password (suggested with dots) and select OK.
Now you can start the service again.