In shifting, my computer has developed ‘disk boot failure’. Insertion of a startup disk boots to the A drive. I couldn’t change to any other drives though. Short of taking it to the repairer could you advise me on how to fix the problem?

I had this same problem when my computer was being shifted. This is commonly caused by cables inside the computer being loose. To fix this, turn off the computer and unplug the computer power cord for the power outlet, take off the case of the computer, touch the chassis (the metal frame of the computer) to earth yourself to discharge static electricity so you don’t ruin the electrical components, and press all the cables running around in your computer firmly (not so hard that you will break them) so that they are definitely in their sockets. Then put the case back on your computer, plug it back in and turn it on to test that all is well.