Problem
You are using VirtualBox with a Linux Guest and the clipboard sharing between Host and Guest stops working.
Solution
Open a Terminal on the Linux Guest and run the following two commands:
killall VBoxClient
VBoxClient-all
References
VirtualBox bidirectional clipboard sharing stops working after some time
If you want to see more about Shared Clipboard on Virtualbox you can checkout Virtualbox Bidirectional Shared Clipboard video tutorial.