SEO expert with 10 years experience. Audits, On-site SEO, Strategy, link diagnostic. I can help you find what's wrong with your website. English and French.
First, gather more details about the lost:
1. Is the page still indexed? (site:yourdomain.com/inner-page-if-it-applies/)
2. If still indexed, are you still somewhere between page 1 and 10?
3. Make sure you don't have a meta noindex on the page or a password, robots disallow, ip based redirection, etc. preventing the bots to crawl the page/site.
4. In Webmaster Tools, try the "Crawl > Fetch as Google" tool to see if they can read the page. If not indexed and they can read, click "submit to index".
5. Look for traffic loss correlation with Google Update (http://panguintool.barracuda-digital.co.uk/). Maybe you've been hit by Panda 4.0 or one of the "Other" updates.
6. Look at your overall organic position recent changes in SEMRush.com to confirm an overall ranking lost.
7. Look for correlation between the lost and any change made to the site in the weeks preceding the lost.
8. Look for suspicious new backlinks preceding the lost with ahrefs.com.
If you don't find your problem, send me a message with your URL and details about the steps above, it'll be a pleasure to take a look and tell you if I can help.
I agree with what was previously said. It will be time-sensitive but worth it if you implement a system to update your sitemap automatically in the future. Also, a solution might exist for your platform that will save you time.
Dividing your sitemap using a sitemap index will also 1. help diagnose possible indexing problems via Webmaster Tools 2. force you to review your site architecture which can lead to valuable insights to improve your site.
Relevant article on the subject: http://www.distilled.net/blog/seo/indexation-problems-diagnosis-using-google-webmaster-tools/