We lost backpack today morning yellow line C, small black backpack. May you help me about that.

Hi Jelena,
For Lost & Found it looks like the regional train authority has now formalized the process of declaring an object lost & consolidated its Lost & Found service through the Gares et Connexions group https://www.garesetconnexions.sncf/fr/service-client/a-vos-cotes/objet-perdu-trouve

There you can register/declare your lost backpack & provide the necessary description & contact information.

If you can remember the line you were taking and its terminus station, you could also try to go to that terminus station and asking the local staff at that station if they found your backpack and still happen to have it / have not yet turned it over to the centralized Lost & Found service yet.  (As most likely your backpack ended up traveling all the way to the terminus station and staff working to clean & turnaround trains there found & processed the item.)
 
Best of luck!