My Bitcoin client has stopped syncing As you can see my client is stuck on 99,7% with unknown. Syncing Headers(577895), It has been in this state since last day synced(which you can see). How can I get it to work since I have plenty of disk space and I run a computer maintenance program quite often.
Take a look at the lower right corner of your "Bitcoin Core - Wallet" window where you see icons of BTC, a lock icon, HD, and two more to the right. The icon on the right to the HD shows how many nodes you are connected to. And, it seems like your node is not connected to any peers.

There might be two causes of this: 


Your node is not connected to the internet. 
There are no other bitcoin nodes available at that specific moment.


For these two problems, you should check and make sure you are connected to the internet, and wait for your node to pick up on other peers on the network.