EXCLUSIVE

Jennette McCurdy -- who plays Sam on Nickelodeon's Sam & Cat -- suffered a tragedy today ... her mom died.

We're told Jennette is devastated -- she was very close to her mother, Debra McCurdy. Debra had been fighting cancer and was very ill.

McCurdy was supposed to host a big show in NYC this weekend -- Nickelodeon's Worldwide Day of Play, where the network goes dark for 3 hours and kids are encouraged to get out and play. We're told she's cancelled the gig.