According to Jud McCranie :
As I understand it, you have 3 wires, two of them carry 240V and the
third is a ground back to the power company.

I wouldn't describe it quite that way. You have a 240V circuit, with
a centre tap (normally called neutral, the fact that it's also grounded
isn't relevant in this context). You get 120V between either hot and
neutral. Each 120V circuit is the neutral plus a connection to one
or the other of the two hots.

Is dropping a leg related to that?

Yup. You lost one of the hot legs. 120V circuits "on that side" failed.
Since you only had one working conductor in your 240V circuits, they
didn't work either. Your dryer spun because the motor is 120V
(and it was on the working side), but the heating element didn't, because
one end of the element was disconnected.

Why would some rooms have electricity and others not, in a situation
like this?

Simply depends on which 120V "leg" they were attached to.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.