Same thing has happened to me. Either this is a hardware issue or specific to Yosemite.
The reason why I mention this as a hardware issue, is that I had random hangs on Mavericks, where the system would freeze for 5-10 seconds several times a day, sometimes more than 20 times a day. Since Yosemite I have not seen this happen, but complete freezes with a hard reboot as the only option, but only twice since I installed Yosemite 4 days ago. As others have mentioned, the mouse is still able to move though.
Testing now if disabling the build-in graphics helps, but as others have mentioned, this is a temporary workaround, as this affects battery life etc.
Hopefully with a lot of reports being sent to Apple, they will get focus on this and get it solved sooner rather than later. Make sure to send your crash report to Apple, after rebooting the machine.
Summary of crash report (I have 19 similar ones of these happening over the time span of around 20 seconds when the system freezes):
Event: GPU Reset
Data/Time: Mon Oct 27 12:10:47 2014
Application:
Path:
OS Version: Mac OS X Version 10.10 (Build 14A389)
Graphics Hardware: Intel HD Graphics 4000
Signature: 801