Ok, let me explain a bit more.
robots.txt isn't needed if you don't want to prevent search engines from crawling your site, indexing whatever to their hearts content. If you want to block certain things or instruct the crawler on what to do and not do, supply an robots.txt file and it will respond accordingly.
The "network unreachable" message means there was some kind of network problem that prevented access to your site, including robots.txt. When you submitted your site to the search engine, you wrote you received 54 error messages.
Those messages might have provided some clues as to what the "network unreachable" conditions actually were.
Your website host might have been having networking problems or there might have been some other kind of problem in the Internet which prevented access to your site.
The robots.txt isn't directly related to the 'network unreachable' error even though the 'network unreachable' error prevented access to robots.txt.
So, if you can find out what caused the 'network unreachable' error at the time you were interacting with the search engine, you can determine how that has impacted your placement other than what you've noticed already. Try submitting your site to the engines again and see what happens.
Peace...