I had a strange issue recently where my Crashplan backup wasn't working, and no closer inspection it looked as though the network was blocking me from backing up. Fortunately, this wasn't the case. The way to check if this is your issue, is to open up Crashplan, and make sure that the screen looks something like this:
If it does, click on
Settings on the left hand side. Then, click
Network in the top bar. Click
Configure next to Wireless Interfaces, and make sure the network you're trying to use has a check next to it. Click Ok and wait. Clicking the
Backup tab on the left should take you back to the first screen, where it no longer says "Network Blocked", and it should get you an ETA for the backup to complete.