MacBook - MacBook 13" new hard drive won't slide in

I replaced my original hard drive as I needed one with more storage capacity. Replacement drive worked fine for several months then crashed. I then reinstalled the original drive while waiting for the replacement of the defective Hitachi drive. I've done all the same steps, but the hard drive will no longer slide in and connect. Seems like there's a cover within the MacBook that's preventing the connectors from linking. Leaving on a business trip on Sunday and need the computer! Any ideas?