Q:

Hard to pedal even on flat street

I have a woman's cruiser bike with pedal brakes only. Everything has been working great but when I took a bike ride the other day I was on a flat street and it was extremely difficult to pedal...why and what can I do to fix it?

A:

Start with the easier things before busting out the spanners!

Check the bike over. Low pressure tyres are very hard to ride, so add air with a pump.
Look for anything wrong, that's anything rubbing or otherwise out of line.
Walk the bike around and make sure it coasts okay.
Rotate the pedal crank while lifting the rear wheel off the ground and make sure it looks right with no bad noises/feelings.

If it is the back wheel not spinning freely even when off the ground, sounds like your brake may be the cause.  A teardown can be a complex and messy thing, how's your mechanical skills?   
Final point - if there is only one brake and its not working right, don't ride the bike till its fixed.  Many locations require two separate braking subsystems.  Where's your front brake gone ?  Also, riding it in the current state may worsen a repair into a replacement.

