It is never a good idea to open a hard drive if you aren't equipped with the tools and knowledge to work with what you find, let alone power it on with the cover off outside a clean room environment.
We get these drive very frequently at my lab for recovery. In cases when the drive was not previously opened and DIY recovery attempts made, we are able to save the original heads and recover the data at our base recovery rate somewhere around 80% of the time. After it has been tampered with, 90% of the time we have to change the heads.
If the data on the drive is worth a few hundred bucks to you, seek professional data recovery assistance. Be aware that many labs will charge significantly more after a drive has been previously opened...