Engine failure after oil change

Help! Changed the oil and oil filter on my 273 in my Barracuda today. Let it warm up for 20 min and then headed to the store. I forgot to check the oil pressure gauge. ( I'm an idiot)

3 miles away from home, the engine suddenly cut out and stopped. No smoke, no appreciable noises (although it is a convertible with the top down and I couldn't hear much). I tried to turn it over, but it wouldn't budge.

After towing it home, engine appears seized- I can't budge it with a socket even. Any thoughts on what I need to do next?