Security Problem on External hard Drive

I connected my external hard disk to a computer logged on the LAN…I right clicked on one of my folders on the  external hard disk…properties…and went to the option…”security”. Here on the lower half of the property window…I checked the option of “full control”….and said ‘Ok”…

Here’s the problem….I blocked the folder….I do not have any access to it…I can’t even delete it….if I go back to properties….the” security” option isn’t there anymore.

I even tried it on my own laptop (not on the LAN) and am unable to access the folder…

Please advice how am I able to retrieve my folder.