Wanted to back up blockchain to save space, now I cannot open bitcoin QT? I wanted to back up the blockchain on my hard drive as it took around 50 GB on my laptop. I did so and deleted all the blk000.dat and the rev000.dat from (~/Library/Application Support/Bitcoin/) which are saved on my hard drive. Apparently this was a stupid mistake as I cannot open Bitcoin QT anymore. I therefore re-transfered everything back to the same folder. But nothing happens, and Bitcoin Core does not open. 

After looking around, many are mentioning a bootstrap.dat file which correspond to the block chain. I cannot seem to find this file anywhere on my laptop.

Any clues? thanks a lot!
Delete your chainstate and blocks folder and start Bitcoin-Qt again. It will then re-sync your blockchain.