Since the salt is consumed by being dissolved by the water in the brine tank and then used up during regeneration, keep that in mind going forward.

Check the most obvious items first:

1) Is the water softener plugged in?
2) Make sure your water softener is not bypassed. Check the built in bypass of the water softener and also to see if there is an external bypass within the plumbing that may be open around the water softener.
3) If your water softener is a metered softener, run some water from a treated faucet in the house to see if the water softener is sensing flow, if not, your meter is not working.
4) Check to see if you have a salt clog or bridge. I have a great video here that shows you how to remove a salt clog.
Run the water softener through a manual regeneration. If it doesn’t start or stops part way through the regeneration cycle, either the motor has stopped working or the piston is seized inside the valve so you would either need to replace the motor, rebuild the valve or replace the water softener.