Hello, I download the new release of RouterOS-x86-3.15.npk into files throught winbox to upgrade my older version which 3.13, nothing happened after restarting. What could be the problem? 
Make sure the RouterOS-x86-3.15.npk file is placed in the root directory. You can also run a MD5-checksum against the file to be sure it was not corrupt during download.
I upgraded 5 routers from .14 to .15 yesterday - 3 of them had issues, 1 partially lost configuration settings.
In all cases the router didn’t reboot properly, I had to hard reset it, and when it came back, only half of the packages was installed which meant I had to re-install the new .15 packages that was missing.
weird… 
Most of our routers are remote - like as in REMOTE..!!! - long way from us and I would never think of upgrading if there was an issue -because “hard-reset” means a LOT of kilomoters on the vehicles…
We upload to our central BSS behind the office and let the new OS run there for a couple of days and then distribute it across the links and then auto-upgrade the Edge Routers from there…
(the clients are done sperately - on a schedule)
Not one gave an issue - not one… all runing rock solid…
I cannot (will not) allow myself to even think of the chaos if “something” went wrong..!!! I think I’d move to Latvia..!!!
Tanker