If your WiFi network suddenly disappeared from the WiFi network list, it could be because you accidentally hid the SSID; or there could be a band compatibility issue. It's advisable to follow the suggestions on both these sections if you're unsure of what the issue is.
Some client devices (printers, cameras, smart TVs, e-readers, etc) only see the 2.4GHz band and will either not see a WiFi network that is only broadcasting 5GHz, or will connect to the SSID when it's broadcasting 2.4GHz, but the moment it pushes the device to the other band because it's clearer, the device will drop the signal.
The solution would be to create an Additional SSID that broadcasts 2.4GHz only and then connect your devices to that Wi-Fi. In theory, if your AmpliFi system's SSID is broadcasting on both bands, your devices should not have any issues, but this is a good way to identify the issue and solve it as well. See this article for steps on how to create an Additional SSID that broadcasts on 2.4GHz: Configuring SSIDs.
Another common cause is accidentally hiding the SSID. Be warned that if you hid your SSID and you have not enabled remote access, you will not have a way to access the mobile app and fix it. If that's the case, you will need to perform a factory reset. If you do have remote access enabled, then you just need to make sure your mobile is connected to the internet (another WiFi network, or your mobile's cellular data), and launch the AmpliFi app to Unhide the SSID. This article explains how: Hiding the Wi-Fi SSID.
If neither one of these two solutions helped, check out this article for more solutions on related issues (for example, your device can see the WiFi, but when connected you have no internet): Troubleshooting: Device Unable to Connect to the Internet. Finally, if none of these articles help, please try resetting your system following the best practices described in this article and set up your AmpliFi system again: Factory Resetting AmpliFi.