This is a huge question and I can only hit a few high points until you tell us more.
Are you drinking enough fluids? 6-8 glasses per day? (some of your weight loss could also be fluids) Your weight loss otherwise should not affect milk supply unless you are taking certain supplements that are contraindicated for milk supply. However, 12 lbs in one month seems a bit high and it may have taken its toll on your body.
Is your child complaining of not enough milk? Pumping isn't a good measure of your supply, in case you were doing that.
Are you nursing regularly, or taking long breaks between sessions (for example, several days away, then back to nursing)? This could have a minor impact on your supply, but a hungry nurser will encourage the milk to return zoo enough.
A few basic solutions:
Fenugreek capsules + Blessed Thistle capsules, daily (there are recommended amounts that you can find on any respectable website - I may hunt one up for you). Then add "Milkmaid Tea" or something similar that is a combination to increase milk supply. It works for many women, especially in addition to the herbs above.
If the above doesn't help your supply within several weeks (you should notice a difference after about a week or two), then seek out a prescription from you GP for "domperidone" - not every Dr will prescribe it to mothers not nursing infants, but you may have a sympathetic Dr. This will do it for most women.
My first choice would be the added water and the herbs.
Here are some good information sources:
(some of them include a lot of info for mothers of infants, but many of the same concepts apply)