A loss of form that has become a crisis. It seems to happen with players when they lose a bit of motivation. Last year, Djokovic finally won the French Open, then he lost early at Wimbledon and in the first round of the Olympics (to Del Potro to be fair to him). He got back into it a bit more later in the year when Murray was chasing him down for the #1 ranking but he still didn't get back to top form and eventually lost top spot and hasn't got a realistic chance of getting it back until at least after Wimbledon.
He dumped Boris Becker as coach at the end of 2016, and the rest of his team earlier this year when form wasn't improving, don't know if that's set him back even more or whether it has just had no effect.