Ubuntu 12.04 applications launching problem

[Geoffrey Wilfred Combes]

This fault occurred a few days ago out-of the-blue, that is, seemingly
unprovoked. It has prevented launching any applications other than those
(with one exception as described below) that happen to be among the
launching icons on the LHS of screen. The two abnormalities are:
1. Dash Home will not list any applications nor find any.
2. Ubuntu Software Centre will not launch from the side icon providing
only a blank screen and then, upon closing the window, leaving a message
saying that it is not responding.
Notes: Dash Home searches for and finds Files and Folders, Music and
DVDs (but not applications).
I have the home directory saved on an external hard drive (I use
Grsync).
By chance I have Byobu Terminal in the launching panel (and it
launches).

My question is: What is the best way of restoring proper operation. For
example, if it is best to upgrade, should I go straight to 14.04 from a
disk or upgrade sequentially to 14.04 as I did to get 12.04 (or doesn't
it matter which way)? Or, can I do something from Byobu Terminal.
Thanks,
Geoffrey Combes