If your retainer does not fit anymore, it is helpful to figure out why. If it does not fit because it has not been worn in some time, then it means that your teeth have shifted. If it has been consistently worn, then there may be something wrong with the retainer.
Regardless of the reason, it would be best to go back to the person who make the retainer for you in the first place as that doctor can properly evaluate whether your teeth have shifted and if there are any destructive interferences (teeth are hitting each other improperly). If this is not an option, you can usually go to a dentist for a free consultation and discuss the different types of retainers that can be made for you. Hope this helps
My best to you!
William F. Scott IV, DMD