If you're trying to login to a Nest (or Dropcam) account at the moment, you probably can't. The company acknowledged a service problem with the mobile and web app on its Twitter account, but so far there are no details about what's behind the problem. Thermostat owners can still change their temperature the old-fashioned way, by walking up to it and fiddling with the device, but that's probably the kind of experience they were trying to get away from. The same probably goes for those with the Nest Protect smoke alarms, although we're hoping no one finds out for sure before things are back up and running. Dropcam users don't seem to have any kind of direct interface workaround for the time being, so they'll just have to wait until access via the cloud is restored.
Update: As of midnight (ET), it appears service is back up, we're able to log in and many readers say it's working for them too. Nest tweeted that service is restored, if we hear anything about what caused things to go down for a few hours, we'll let you know.