I lost my retainer a couple months ago, but I removed my braces 6 years ago. I see my teeth have shifted! What do I so since it's been so long?

Minor corrections? If it's insignificant movement and does not interfere with your bite then minor corrections will fix the problem. However if the shift is serious the limited orthodontic treatment is necessary. Once the tooth is alined it has to be in good contact with the adjacent teeth and in harmony (bite) with the opposing tooth. Rec.: to see orthodontist for final evaluation and correction.
Return. Return immediately to your treating Orthodontist or see a new one. As you can see delay is not your friend. Before it gets worse see an Orthodontic Specialist for options.
Lost retainer. You may or may not get further shifting of your teeth over time. The best scenario would be to see an orthodontist asap (either the one who treated you or if unavailable another). At least get a replacement retainer. Better yet, discuss correcting the shifting prior to the new retainer. If it's only been a short time, a simple removable appliance will probable straighten your teeth in no time.