One of the frequent problems with the Sinclair computers of the early 1980's in the keyboard membranes turning brittle. This results in loss of one or more rows or columns in the keyboard. These are split into 5 rows of 8, so if some columns are out, you may have keys working and some not. For example, keys 3,4 and 5 working and also 6, 7 and 8, but 1,2, 9 and 0 don't respond, or if rows are out, 1,2,3,4 and 5 work, but 6,7,8,9 and 0 don't etc. It's rarely a single key. In this case, it's all of them.
USB keyboards. I've designed the boards to be able to cope with short membrane tails that would otherwise be to short and would have to be replaced.
RWAP) if yours is a bit scratched or dented. Remove the existing adhesive or tape, clean well and apply new double sided tape.