Remove it from the Macbook and have a look at things that may of gone wrong like screws in the right place ect....
If you re fit it again and get the same issue i have a good youtube video for you i recently watched which is very good just to make sure your applying correctly
Take a look at the hard drive, which you removed. It should have 2 torx 6 screws screwed into either side (for a total of 4) of the hard drive. These 4 screws need to be removed and replaced on to the new hard drive prior to remounting.
Take a look at the hard drive, which you removed. It should have 2 torx 6 screws screwed into either side (for a total of 4) of the hard drive. These 4 screws need to be removed and replaced on to the new hard drive prior to remounting.