Old retainers may still be usable
Anyone who has had orthodontic treatment, either with traditional braces or with clear aligner therapy (such as Invisalign), had a retainer made after treatment in order to keep the teeth properly aligned. Most patients that do not wear their retainers will soon discover that their teeth have shifted, and the old retainer might not fit anymore. Others may find that the old retainer still fits, but is very tight and uncomfortable.
If the retainer is tight, we recommend wearing it anyway, even if it makes your teeth a little sore at first. After a few successive nights of wear, patients will find that their retainer fits much better and isn't as uncomfortable. This is because the teeth are moving back into their ideal positions, as guided by the retainer.
A new retainer may be needed
If the retainer doesn't fit at all, a new one can be easily fabricated. The new retainer will keep the teeth where they are, but it will not be able to move the teeth where they are now, but it will not be able to move the teeth back to where they were at the moment the braces were removed. In order to properly align your teeth, additional braces or clear aligner therapy (such as Invisalign) would be necessary.