Gnucash backends

[Jonathan Silvey]

I have been using GnuCash on Linux Mint for several years. As they were
getting full, I decided to start a new file and this has been running
successfully for three weeks. When I try to run the old data, the relevant
files are there but when I click on them, I get 'No suitable back-end found
 ...'

  I can't find out on Google what a backend is and I don't know how to load
a
 file. Can you help, please?

 Jonathan