It is likely that when you reset your router, it has allocated a different IP address to your mycloud, using DHCP , unless you have taken steps to prevent this (setting an infinite DHCP timeout at the router, or ‘always use this address’, or setting a static IP address using the router).
You said you can see the MyCloud on the network. What IP address has the router given it? Then try to access the Dashboard using the IP address, rather than the name.
You may have to turn off your PC and sonos to get them to figure out the changes to the network, or go through the network configuration process with the sonos again, to tell it where to find the MyCloud, if its IP address has changed.
If you MyCloud was working fine up to this point, I see no need to upgrade the firmware. It was probably just a router crash, and resetting has recovered that, but left your network devices a bit confused. If you want to know how to o firmware upgrades, and generally use the MyCloud, it would be helpful to read the user manual:
http://www.wdc.com/wdproducts/library/?id=439&type=25
If you want to understand and set up the Twonky media server running on the MyCloud, so that it survives issues like this a bit better, you might read this FAQ.
