Why would I have little to no water pressure after reopening the main water valve?
I would not count on the main valve - it could have broke. I had similar situation last year, but in my case it was that i couldn't close the main valve. If every output - even the ones that You didn't touch - gives same response (little or no pressure), it is highly probable that main valve broke down.  

I would suggest calling city plumbers (as this is main valve) to check and replace it.