I'm sorry for your misfortune, but the Avis location information for Concord Airport indicates the following:

"General Directions


"AIRLINE PASSENGERS: Proceed to the Avis counter in the baggage claim area. Courtesy bus service is available between the airport terminal (outside the baggage claim area) and the Avis car rental lot.


"WALK-UP RENTERS: Follow airport signs for Car Rental Return. The location is on Zephyr Pl at the terminal, between I-85 and Delta Rd, north of Concord Mills Blvd." (Emphasis added.)

I'm not sure why Avis does not post a sign at the in-terminal rental counter with this information.

It appears that Avis also has an-off airport location in nearby Kannapolis:

"Concord Car Rental & Nearby Locations1 Cloverleaf Plaza 3.26 miles awayAddress:

934 Cloverleaf Plaza

Phone:

(1) 704-792-9009

Location Type:

Corporate

Hours of Operation:

Sun 10:00 AM - 1:00 PM; Mon - Fri 9:00 AM - 5:00 PM; Sat 9:00 AM - 1:00 PM"

Better luck next time.

Edited to add: Avis also has several off-airport locations in the Charlotte area, which might be more convenient for you -- unless you are trying to rent a vehicle type only available at airport locations.