There's a post in seroundtable regarding a question about the loss of index pages during a temporary server crash.

Based on my experience, it depends on the total time your website is unavailable. I don't think google or any search engine will delist you if your server went down for half a day. But for longer periods of unavailability, you might see less pages.

I had experienced this before. I had a site which I didn't often visit. My webhost somehow deleted my SQL database so my site is just showing error messages. It was probably a week or more before it was restored and I saw my number of pages indexed drop.

I strongly believe that google will somehow delist some pages that are inaccessible. I haven't seen any test done yet on how many days before google delist those pages but knowing google, its Primary goal is to provide relevant results. If you site is down for a long period, to include your pages in the search engine results pages is "not relevant" to the searcher as your site cannot be reached.