Gary Campbell, who keeps a register of sightings, said no-one had come forward in 18 months to say they had seen the monster.
Bookmaker William Hill also said the three entries to its annual Nessie spotting contest could be explained.
They were images of a wave, a duck and a picture not even taken on Loch Ness.
Mr Campbell said it was the first time since 1925 that there had been no confirmed reports of the monster.
He said: "It's very upsetting news and we don't know where she's gone.
"The number of sightings has been reducing since the turn of the century but this is the first time in almost 90 years that Nessie wasn't seen at all."
Mr Campbell, a chartered accountant based in Inverness, has been logging Nessie sightings for 17 years after seeing something in the loch himself.
As Nessie's registrar of sightings, he has put together a list of sightings going back 1,500 years.
Irish missionary St Columba is said to have encountered a beast in the River Ness in 565AD.
BBC News - No Loch Ness Monster sightings for first time since 1925