I always work with many virtual machines that I never close and always save the state of the machine instead of closing it.  

Many times I find that I lose the clipboard between the virtual machine and the host, making it impossible to copy and paste. Being a problem  restarting the virtual machine.  

The solution is to restart the virtual box program in the virtual machine that manages the clipboard or all process client of Virtual Box by executing the following command as root or user that has permissions to execute them:

killall VBoxClient

I have certified this in a CentOS 7