I had an overbite & had braces to fix it, I lost my top retainer, do I absolutely need another?

It is necessary if you want to keep your teeth perfectly straight.  Depending on how your teeth looked before braces, teeth always try to return to their original positions especially in the first 6 months

Yes. I strongly recommend you to get another retainer as soon as possible. Your teeth will relapse, i.e. slowly going back to their before-treatment position. How long it will take for the relapse to occur varies from individual to individual. After 3-6 months full time wear, retainers are to be worn at night time indefinitely. Consult with your orthodontist.