Patents expire. Patents issued after June 8, 1995 expire 20 years from the date of filing; patents issued before that date expire 17 years from date of issue. So, for example, if your father's patent issued in 1991, it would now be in the public domain. That's not to say you couldn't bring a claim against someone for actions that took place while the patent was active but that's a very tricky type of claim to bring (and the clock is ticking).
Patents are assigned. We're not sure of the situation but it sounds like some funny business occurred regarding your father's patent, his company, and his employees. If your father transferred his rights in the patent to his company, that information can usually be located by searching at Google Patents or the USPTO. Use the Advanced Search features, put in your father's name as inventor and find the patent. Check the dates to see if it has expired and look to see if an "assignee" is listed. If so, the assignee owns the patent rights. That doesn't mean that your Dad gave up all revenue. The assignment may have been an agreement promising him payments. So you would need to find the assignment document to see what the terms were. You can also search assignment documents at the USPTO here (in case the assignment was filed at a later date than the patent). Whatever you do, you should act quickly, as the passage of the ten year period since your father's death may have caused you to lose rights.