If i haven't worn my retainers for a few months, will they shift my teeth back into place if it still fits?

With luck: Depending on how your teeth have shifted, this may be possible. If the space required for proper alignment is not still available, however, it is much less likely. Go see your orthodontic provider with your retainer to get information that is relevant to your situation .

Possibly: They may also damage your teeth, gums, and bone if too much pressure is placed on the teeth. Advice would be to see your orthodontist.. bring your retainer to your visit.

Most likely: If your retainer still fits you can start wearing it and you should see improved alignment soon.