Gas furnace-5 amp fuse keeps blowing…help please?

i bought a home a little over a year ago. it has a a brand new gas furnace and condensing unit. over the summer i had to replace the romex wire going out to the disconnect box for the condensing unit because of a lightning storm. once i did that everything was fine. well then fall came and i didnt need to run either furnace or ac. then the first night it got cold i went to turn the furnace on and it popped the 5 amp fuse on the furnace circuit board. So, now any time i replace the fuse and turn it on to heat it instantly pops the fuse. if i turn the ac on it doesnt pop the fuse but doesnt run either. i thought that it would be the transformer because of nothing running at all. I am not up to date on these new furnaces. i have worked on numerous manuel furnaces without all this circuit board jumble so its really just throwin me for a loop. please someone help me out