My experience, for what it is worth.
I had a similar problem with a Dell e6420 laptop, purchased used with Windows 7, and Ubuntu 14.04 added after the fact by me. The touchpad was turned off, and a Logitech wireless mouse with 'Unifying receiver' used. The primary application is as a Minecraft gaming machine.
After apparently random periods of time these symptoms would appear: the mouse pointer would move, but the mouse buttons and keyboard enter key would stop working. The only consistent work-around I found was a power reset. The quite extensive embedded Dell diagnostics reported no hardware issues.
I noticed that these symptoms primarily occurred with a userid set up with restrictive permissions, and configured to auto-login on boot. Using the "Users and Groups" administrative application (sudo apt-get install gnome-system-tools) I selected the offending account, hit the "Advanced" button, selected the "Permissions" tab and ensured that all check boxes were ticked (In my case, I added ticks to the only three that were not previously selected: tape drives, floppy drives and send and receive faxes. To my mind, the three have no apparent link to mice or keyboards).
With this as the only change, after a reboot the symptoms disappeared.