VirtualBox/Ubuntu mouse disappears

One suggestion I found was to "Just disable "mouse integration" in Machine tab.". Also on that page "Moving the Vbox window to my primary screen solved all issues." You can also toggle mouse integration from the lower-right corner of the guest:

I also wonder if this is related or would be of help

The vboxvideo driver causes the mouse pointer to disappear

There is a simple answer to this. VirtualBox latest guest addition 4.3.10 are still not fully compatible with Ubuntu 14.04. Screen auto-resizing issue, mouse pointer issue, shared clipboard issue and there are some others that occur intermittently with 14.04 VM.

Use Precise (12.04 Ubuntu) or wait for a new release of VBox to solve this issue.

I had similar issue (i.e. my mouse never showed up at all). None of the above solutions worked for me. What worked for me is select the VM by clicking and highlighting it in the VirtualBox. Then navigate to the "Settings". Click on the "Display" in the left pane. Then inside the "Video" tab, un-check "Enable 3D Acceleration". Apparently I had clicked on this checkbox that made my mouse pointer in VM disappear. Now boot the VM.