The disavow tool is a helpful Google Webmasters Tools feature that allows website owners to submit their site’s spammy links to Google in an attempt to remove the bad links from Google’s ranking factor. With poor links going to your site, Google may penalize you for spammy link farming. However, sometimes people that are novices with the tool may go overboard and disavow too many links. Fortunately, Google offers the ability to re-avow the bad links leading to your website. If you realize you need to go back and reavow a link, you simply need to resubmit the disavow file you used before, but with the good link removed from the disavow file.
However, when Google adds the link back to your site’s citations, it might take a delay before its complete. In order to avoid having webmasters game the system, Google delays including the link back into your citations. Additionally, the link may never have the same power as it did before. So while reavowing is possible, you should be careful disavowing links in the first place, and consult a professional if you need help.