Google shows unreachable URLs when it encountered an error when trying to access those URLs. for example: It might be a DNS error or timeout. Your server may have been down or busy when Google tried to access those pages. Possible URL unreachable errors include: 5xx error DNS issue robots.txt file unreachable Network unreachable Failed to connect The server closed the connection before we could receive a response etc. Hope this will help.
yeah it happens when your server restarts etc. I restart my server once per week and every time google catches a url, its like that site that says when your website is down.
Create an up to date XML sitemap and resubmit it to Google, as well as a URLList.txt that will help... Check your inner linking structure to make sure it's not pointing to any unreachable URL's and fix it... If you have backlinks pointing to those URL's simple ignore it