Terminal & password problem

[Bumpalot]

Kubuntu 14.10 OS Type 64-bit Processors: 4XAMD Athlon II X4 645 Processor 7.8 GiB Memory

Have had to reinstall Kubuntu 14.10 from CD early today.
Have been using computer for 5 hours, including the Terminal, then suddenly..
Problem 1:
1. on opening Terminal – tried to run sudo apt-get install make
--would not accept my password (which has worked OK all morning!!)
Question: how can I change my password

Problem 2:
open Terminal again:
-without typing anything:
Popup Msg:
Sorry - Plasma Desktop shell:
KDEInit could not launch '/usr/lib/kde4/libexec/kdesu'

Any ideas how I can fix these?

[Bumpalot]

Tried that several times.
It's also acting very strangely - a few moments ago, everyting was frozen - Kickoff, all shortcuts.. Now I have updates, but my password is "incorrect"!
At least, for now, how can I change it?

Don't bother answering! Chaos reigns, have to re-install..