I am sorry to learn of this situation.
As you do owe dues (and late fees), but they failed to give proper notice, you have some good room to negotiate.
You can hire a lawyer to represent you if you don't have any luck working with them directly (if you are pressed for time - meaning, if the sale is coming up very quickly you may want to hire a lawyer anyway - it isn't worth risking a sale date to save a few dollars, you can try to negotiate the cost of hiring an attorney into a reduction of your fees, but I wouldn't recommend messing around on this one if the sale date is approaching).
If the sale date is a ways off, start by sending them a letter, acknowledge that you owe assessments and fees, but identify the fact that they failed to give notice, give them a short period of time (for example "10 days from the date of this letter") to acknowledge your letter and remove the sale date, then remove the lien, at which point you will pay the past due assessments. (Or something to that affect - I am only giving suggestions for ways you may handle this, I cannot advise on specific courses of action).
You can also try using a mediator - a third party neutral is a good way to help break through an impasse and help negotiate a resolution in this type of situation. Contact the local bar association and ask for referrals to mediators - the mediator will contact the opposing attorney and help set up a mediation to resolve the issue (notify them of the time constraints at issue).
If none of this works, you will need to file a motion with the court to enjoin the sale (get an injunction based on the failure to give notice), but again, if you are at this stage, please hire a lawyer, I would hate to see you lose your house in an auction based on missing a procedural issue or some other matter (it is very hard to self educate yourself quickly enough to make effective use of the legal process - hiring a lawyer doesn't give you any additional rights, but it does help you fully exercise all of your rights).