My wired aluminium keyboard won't type "i,j,m,u,space,1,2,4,5,7

I have a mid 2009 iMac running 10.11.6 (El Capitan), everything apparently OK, until yesterday when the keyboard stopped producing the letters and numbers in the heading. At first I thought I'd been the victim of ransomware, but everything works OK using an old Tesco wired keyboard with a wired mouse. Any ideas as to what the problem is?