I am running ToA and all the gmnotes that came pre-loaded in the game on have vanished!
Such as on map marker tokens, they used to have the description from the handout of that area on the tokens gm notes.
They were there just a few days ago now they are all gone.
GMNotes (and Notes and Bio fiels) are only loaded when you first open the handout or token. There might be a network issue or a slow shard that timeouts and prevents the browser from loading the notes. It happens quite frequently to me too. Closing and clicking again (after some time) should help.
Via http://status.roll20.net/ you can see that the average responsetime of the shard is momentarily around 400-500 milliseconds, so with a text with embedded pictures and some network hickups those halve seconds start to add up.
If this occurs: never, ever change the item and/or save it! It will overwrite the item in the shard/database and you lose it.
You should report this in the Wizards of the Coast Bug Report Megathread. That thread is monitored by the devs and you will have a higher chance of being seen.