I have some left over ganache that I used last month that I had put into the freezer. Can I reuse the ganache as a filling in a cake? If so, would I need to do anything special to it?

Sure can - just take from your freezer and allow to come to room temperature - or just leave out (covered) overnight.
Then give it a good stir - and it shall be ready to use for your cake.

If i know i am going to be busy in the coming weeks- i make it and freeze it ahead so as to save time...plus it is always so handy to have some on hand.


Bluehue

Agree with Bluehue, but if it still seems too firm after you've done that, you can beat it in the electric mixer and it will fluff up again. I do it all the time.