Notification Center frozen in macbook pro with Yosemite 10.10.3
The simplest and least destructive way to stop processes that run in your user space is to logoff, and logon again.

If you kill a process, it can't save any data correctly. Most system processes have good routines to handle this, but that slows things down the next time it starts and can leave things in a state where you need to clean up the mess by hand.

Logging out and back in will minimize the risk of causing further issues.