Magento is relentless and notorious for error like this one I got on admin and with not an accurate fix for it. Is there anyone here came across this issue and why this happen out of the blue.
There may be three reason for this:
1) CMS Module Disabled
the CMS module handles 404 not found requests which is the fallback router. So, no router may be found if this module is disabled. Just enable the CMS module and check if the issue is solved
2) Cache is Corrupted:
In this case, the list of routers never gets loaded because it loads from cache and the corrupted cache does not contain this data. As a result, fully flushing the Magento cache will resolve thi issue. You can also delete /var/cache folder to remove cache corruption
3) Check that your Magento installation knows the default no-route url. Navigate to System > Configurations > Web > Default Pages and locate the input field for Default No-Route URL. Make sure that this value is set properly to the default value of cms/index/noRoute.