Ok so it's only the mouse that's wireless but once the routines that
service the mouse/keyboard lock up then plugging in another mouse
isn't going to be detected.

The root problem is what's interfering with the wireless side of
things and this can be any number of causes as mentioned. Even
something as stupid as a taxi driving past that's got a badly set up
transmitter.

Question: Has there been any new mobile phone masts set up nearby ?

Like all random events it can take some time and lateral thinking to
pinpoint the cause ! Meanwhile, try positioning the mouse receiver
nearer the mouse itself if you can. If it's one of those nano-USB type
of receivers your options are limited but running a USB extension to
get it nearer would work.