devtrackers.gg
Discord sometimes has trouble loading servers that are extremely large, so outages may occasionally occur resulting in the server disappearing temporarily. If you scroll to the bottom of your server list when that happens, you may see a red icon with message explaining it.
Like AlphaboyZz and Andrzej667 mentioned, a restart will fix this issue 90% of the time, but even when it doesn't the problem typically resolves itself after a few minutes.