Why did my site vanish from Yahoo and Bing search results?

Common reasons search engine result page results may differ from day to day, week to week.

• Competitors are doing more aggressive SEO
• You have been penalized for linking to bad websites, or joining a link building farm
• You are trying to game the system with hidden text, white text on white backgrounds or other black hat SEO practices
• The Search Engine altered their ranking algorithm (which they do)
• You were, or are logged into Gmail, Hotmail, Yahoo mail which may be localizing or altering your search result positioning.
• Localization is happening and altering your search based on geography
• If you are running a CMS any upgrades that may have altered your pages or URL structure

To best keep track of your SEO positioning use something like Raven Tools

Other than black-hat SEO techniques as mentioned by @Ryan Doom; if you have not practiced any such techniques two months before when you last updated it then consider both on-page and off-page optimizations to help you get back your lost rankings in search results.

On-page optimization

1. Make a competitive keywords research and short list those keywords that you want to target.
2. Implement these keywords (keyword strings) within the page title(s).
3. Use the same set of keywords in the initial (starting) paragraph of your web page(s) that you want to be appeared in search results on those competitive keywords.
4. Use the same keywords as page headings
5. Keywords density also matters so repeat the same set of keyword(s) on your web page(s) for twice at least.
6. Use the keywords on some other pages of your website and employ anchor-texts on these exact keywords where the reference URL should be your main (target) page that you want to appeared in search results on those competitive keywords.

Off-page optimization

1. Heavily but ethically follow the 6th suggestion in the above section while making any submissions on external websites.
2. Since you are into softwares business, so there are plenty of 3rd party directory, forums, news and forum websites where you can promote your products. If you are allowed you reference any URL to support your submission then reference your web page(s) that you want to be appeared in search results.

But above all, let me tell you that Google ranks search results based on relevance mainly. There are plenty of factors too but relevance to the search query is the prime factor. So while doing keywords analysis and research; keep relevance in mind and target only related keywords on related page(s) of your website. This will help Google to choose the best matching page from your website and rank it in SERPs on relevant keywords.

Your competitors have done well in those two inactive months of yours.

You need to build up your reputation with Google, yahoo and bing again.

Search engine just display those who justify those keywords, if your webpage justify it by a good active updatation. I am sure you will be back in business.