Can't find or access external hard drive

Greetings from Australia.

My son uses his mac for video editing and is in the middle of a major dvd project with deadlines looming. Last night while I cleaning up, he was out but his mac was burning a first cut of the project for the man who has hired him, when the burn was complete, I saved everything and then powered down his machine. He has an external hard drive connected as well and because the computer was powered down I thought it would be ok to disconnect it to tidy up my table area. I couldn't see a power button on the drive so I just pulled the power cable out of the back. I was a bad mother but I didn't know it at the time. Right now when he turns on his computer, he can't even see the external drive let alone access it. Have I caused irretrievable damage or can the data be recovered? I'm feeling pretty bad at this point & would appreciate any help or advice you can give me. Thank you in advance for taking the time to read my question.