Aspect ratio stuck at 4:3, missing options for 16:9

I've had Cox TV for right at a year now--I've been doing the plan they offer by which I get my local and HBO channels in HD for like $10. Haven't had any issues until a little earlier this week. I turned on my TV, and everything on every channel has a 4:3 aspect ratio, with the black bars on the sides of my TV.

I haven't changed any settings, have not toyed with my cable box or any of the cables. I went into settings on my cable box and set the video out put format for 1080i Wide. Under aspect ratio options, I can only choose between 4:3 Sidebar/4:3 stretch/etc. No 16:9 aspect ratio. Also, the picture on my screen in the 4:3 is definitely not 1080i. It looks like I'm watching TV on one of the 4:3 box TVs from the late '90s.

I called tech support twice, and they were able to do nothing for me--just set me up with an appointment with a technician.

I'm wondering if anybody else has had any trouble with this, or knew of any solutions. For what it's worth, I have a Samsung Smart TV, model number UN50H5203AF. My cable box is a motorola DCX3400-M.

I've rotated HDMI cables, reset my router, reset my cable box, plugged and unplugged the cable cord at both ends, all to no avail.