A file photo of a meal being prepared at a Chipotle restaurant.

It has been exactly a year since news of an E. coli outbreak accelerated Chipotle's downfall from a Wall Street darling, and investors are wondering when, if ever, the stock and sales will bounce back.

"The bloom's off the rose," said Bob Derrington, senior restaurant analyst at Telsey Advisory Group, who has a "hold" rating on the stock.