Yes, Juana's is closed, permanently from the looks of it. I wouldn't consider it a huge culinary loss but it was real home cooking with a few surprisingly good dishes.
I know Concordia in the heart of La Villita offers (offered?) a lunch buffet. I never tried it and am not sure if it's still available. Call first.
A fairly new restaurant, Merendero San Marcos, currently has a weekday lunch buffet for $6.99 (beer specials on Monday and Wednesday). I haven't eaten there since new owners took over (it used to be La Casa de Don Andres). It's now run by the Gorditas Aquascalientes people so I'm cautiously optimistic about the food.
Juana's (closed)
2406 S Western Av
773-523-4166
El Concordia
3801 W 26th St
773-521-4095
Merendero San Marcos
2120 S Cicero Av
708-652-7822