First of all this is not such a significant backlink loss. I have seen worse. My husband does SEO work and he says that the number of links a SE indexes can vary from time to time, usually fluctuating a few hundred links which seems to be your case. I think you should simply double your SEO efforts and hopefully soon enough you will see the number of links increase. In case you are not sure as to which are the Off page SEO techniques here are the most commonly used.
Forum and blog posting or commenting.
Article writing and distribution
But... thinking about it again, I just realized your signature says you provide SEO services. Shouldn't you know that? Maybe the answer is right in your signature...
What you need to do is to make sure that you are getting more links then the ones being deleted. It might be due to website going down, comments being removed etc. Discipline yourself to post a few articles or get 30-50 links a day.
Luckily, increasing backlinks to your site is very easy, but something that typically is totally overlooked. So many bloggers worry about why their posts/articles don’t receive any comments… where in reality you should be only worried about commenting on other blog sites. Commenting on other sites is very important towards increasing backlinks to your site.
It's better to check your previous postings. Try to check those places previously linking back your site but now you're not getting any links from them. Don't waste much time for it as it may takes long time to check all those sites. Try to continue this job along with your regular submission job. Never stop it. Simply continue your posting jobs specially in niche sites, blogs, forums, social bookmarking sites, articles and what else you think right for your site. You need to take care about your visitors also. So don't post in only do follow sites. Also some no follow sites are good from visitors perspective.
The absolute best way to improve "backlinks" is to improve the content of your site. In other words, if you have an amazing resource, product, or service AND you generally provide a good user experience. Others will naturally cite your website.
Commenting is the key to increasing backlinks. For every comment you leave on another blog site, eventually this will count as a backlink. It does take time for Google to account for the link and attribute the backlink back to your site
The number of links can vary from time to time. You can check with google to see if there is a problem with your links/site. You can also find out which links are missing and this may give you an insight into the problem. But ultimately you want to keep adding more links.