One day, mapped drives to my NAS stopped working and the NAS itself just disappeared from my network.
My first thought was either the NAS broke or my router. But my router seems fine, so I first checked my NAS by directly connecting to it via ethernet cable to my laptop (using USB converter). I also downloaded the Synology Assistant software which helps a lot in finding whether there's Synology NAS in the network.
The Synology Assistant can be found in Synology Download Center under Desktop Utilities. Synology NAS model required to find the right software.
https://www.synology.com/en-us/support/download