Update: All affected Google services appear to be back up now.
Google reported service disruptions for Hangouts, Talk, and Sheets Monday morning.
Google Sheets went down about an hour ago but seems to be recovering gradually at this point, with some users regaining access while others remain deprived. Google Hangouts and Google Talk appear to have gone down a little earlier than Google Sheets did. As of this writing there are no further updates on their status.
But since you might suddenly have a little downtime, you can follow their roads to recovery on the company's Apps Status Dashboard.