I don't know how did you set up your PC, but we are going on the block 377,000 . So, there is no way you could mine a block 680-something. My best guess is that there is UDP port blocking on your hub,router, or firewall and you didn't sync with our network at all, and mined a local blockchain. When you start `geth` it must first synchronize with the full blockchain, and only then you can start mining.But I don't think you would be able to mine with a PC right now, the difficulty rose a lot overnight. We have explorer setup on our website, you can verify all the blocks. If you are mining, they should appear on the explorer.