Can I be dismissed after I have already given my resignation?

I had made intentions to resign my employment from my employer in December of 2016 as bullying towards me was relentless. On Febuary 21, I was contacted by and told that internal investigations was looking into allegations made against me. Nothing too serious. I notified my union of this immediately. I had been unwell and my union representative notified management and HR that as of February 22, I will be going on no pay sick leave effective immediately. As per the employer's request I surrendered all work and related items, and essentialy had no ties remaining with the employer on Febuary 22. On Febuary 24, I successfully sent a letter of resignation effective immediately to my employer and ensured I notified HR and the union and turned in my access cards. I am now being told that my resignation may be ignored and I may be dismissed instead with cause. Is this possible? Can my resignation be ignored and my unblemished record be stained instead with a dismissal? What are my legal rights? Where can I go for help? My union has not been helpful. All I want to do is sucessfully resign, work elsewhere and move on with my life.

Please help.