There is not much you can do now other than wait and cross your fingers. If you make a small cut/deep scratch low on the trunk and see living green tissue there may still be hope, otherwise not so much. It may help to make a tent out of a plastic bag to create a mini greenhouse of sorts. Do not keep it too wet, that will do no good now.
I'm supposed to water it every 2 weeks
This is terrible advice. Depending on species, location, season of the year and type of soil there MAY
be instances when a tree need not be watered for extended periods but to make such a definitive statement is foolhardy and your tree has suffered the consequences.
IF your tree really took two weeks to dry during the summer then that indicates that the soil is entirely inappropriate. Proper bonsai soil drains like a pot full of gravel and the wet/dry cycle should occur much more frequently especially during summer.
What species of tree is it? Where are you located and how has the tree been kept WRT location, inside/outside, it is hard to make meaningful comments without basic information, Please post some pictures if you are able.
Also take a look at the general growing sticky located at the top of the page for basic watering information.