If I had an apartment with a roomate but her parents moved in and changed the locks and packed my stuff without me knowing, can I sue?

Yes, you could sue, though the amount of monetary compensation you could obtain is limited by the extent of the monetary loss, costs,or out-of-pocket expenses you suffered; the legal system only provides compensation for actual, provable losses caused by another party's wrongful behavior. Note that such costs could include hotel/motel costs (if you had to check in somewhere) or the costs to move or store your belongings.

However, you could also seek a court order (injunction) directing the roommate and her parents to change the locks back (or provide you a key to the new locks), give back to you any belongings of yours they are holding onto, let you back into the apartment, for the parents to move out, and to not do this again. A joint or co-tenant (e.g. a roommate; someone with whom you are renting from a landlord) has no right to evict you; and nor do people, like your roomate's parents, who are not even on the lease. Only the landlord has the right to evict you, and only if you give him or her good cause, such as by not paying rent or violating the lease--there is no doubt but that a roommate who is not also you landlord may not move new people in instead of you, change the locks, or pack up your belongings. Thus, based on what you have written, you seem to have a cause of action.