Every so often I run into a Chromecast user who says they can't get their PC to find their Chromecast. More often than not, it's due to a bug in how the Chromecast plugin handles multiple network connections. The Chromecast plugin assumes that your primary active network connection is on the same LAN as your Chromecast dongle. Obviously this is not always the case, especially in the case of VPN users. What you have to do is move that connection to the top priority in your network connection list.
Here's how you fix the dreaded "No Cast devices found" issue. This is for Windows 7, but probably works with 8 and Vista as well:
1) If you are connected to a VPN, disconnect it and try detecting the Chromecast again. Still not working? Proceed...
2) Go to the Network and Sharing Center
3) Click "Change Adapter Settings"
4) Click the Advanced menu and select "Advanced Settings..." (Alt+N, S)
5) Find the network connection that shares the same LAN as your Chromecast and select it
6) Repeatedly click the green up arrow until that connection is at the top of the list.
7) Click OK to close the dialog and apply the new connection priority settings
8) Try detecting your Chromecast again
If you use a VPN, you may be wondering why you have to disconnect. The answer is that many VPN clients manage the connection priority of the virtual adapter they create and automatically moves it to the top of the list. It's best just to disconnect and try again instead of battling that behavior.
Good Luck! I hope this helps someone out there!
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