All your SharePoint sites stopped working abruptly?
The reason is more likely to do with some changes to the application pool identity.
Check if your application pool doesn't start. This can be due to the credentials used for the application pool. Most of the times, on the development machines, we use our own credentials for the identity.
Ideally, the credentials for the application pool must be the system account that doesn't change.
In case, if our passwords are configured to expire after a set period, we need to remember to change this credentials on each of the application pool we are running our sites on.
Also, the credentials are possibly cached on the system and hence it might be necessary to restart the system.