What is my status as a green card holder?

My family emigrated to the States from HK as green card holders in 1999 when I was 19 and I returned to HK after just two months via Canada (and that was before 9/11 and so my exit from the States was not recorded) to finish my degree. I have never returned to the States after that. What is my status as green card holder? Is there any chance I could study or work in the States again and how am I supposed to do that? Will I become subject to any criminal liability? I was a teenager back then and did not really question the propriety of the arrangement mentioned above, which was something my dad came up with.