MacBook – External HD Accidentally pulled out of computer

What is the exact make and model of external enclosure you are using? Some of them require an external AC adaptor. Some of them come with a duplex (two-headed) USB cable and require that both of them be plugged in to powered USB sources to provide enough voltage to run the enclosure and the drive.

Of course it is possible that your hard drive itself is damaged. If that is the case, you will need to pay a lot of money to a specialty hard disk data recovery service.

Finally, do you not already have a Time Machine backup of your Mac's internal hard drive to an external disk drive? When this current situation is resolved, it would be wise to invest in an external backup system using Time Machine.

UPDATE:

Have you tried this drive in a different external enclosure? until you do that, you can't rule out that it's the external enclosure's problem and not the drive's problem.

The first recommendation is to run Disk Utility to repair the external drive. If it is a simple corruption Disk Utility can occasionally fix it.

The next step from that would be to run a higher-end data tree repair utility, such as Disk Warrior. This software costs a bit of money but is better at repairing serious directory structure issues such as this.

You could also attempt a software-based data recovery using a program like Data Rescue. This software is just a bit pricey though, and at some point you will want to consider how valuable this data is. For instance, if it's a simple matter of re-downloading purchased movies or re-ripping DVDs it may be worth saving the money.