You put in over 40 hours and federal overtime requirements kick in. Demand your backpay and/or file suit. You need an attorney specialized in employment law to help you but yes, they owe you money bigtime.
Check into the benefit plan terms, by getting the summary plan descriptions (SPDs), but it sounds as if you would be entitled to your benefits. For some benefits (health, life insurance, disability, etc.), the employer can define the eligible group in almost any way it wants -- "full time" = 50 hours per week, or whatever. For other benefits (pension, profit sharing/401(k), etc.) there are legal standards that have to be met.
If the question is simply one of service, then 1000 hours paid time in a year = a year of service, and you generally cannot be required to complete more than 1 year of service for eligibility.
However, there are other issues that could apply. Certain job categories or work locations may be ineligible, regardless of whether they are full- or part-time. That kind of thing you will have to find out from the documents.
So get hold of the SPDs, see what they say, and, if necessary, consult with a local attorney. You could very well be entitled to payments (to say nothing of the overtime mentioned in the other answers).