Originally Posted by Patty Up North

Depending on the amount of force you put into your effort, trying to install them wrong, you might have damaged the crank arm threads. Try to give your local bike shop a call, they might have a tap in the right size for "opening" the crank arm treads again.
Chek what size your pedal treads are (proberly 9/16"). See if you can find some cheap taps on Ebay, remember you need both left and right. But check if your crank arms are worth the tool investment... Will you need it in your garage in the future?

Or call your local bike shop.