keyboard does not work

we have a pentium 1 machine and the problem we are having is…the machine switches on fine with no error messages ,but when we try to use the keyboard we cannot get it to work,the numlock light stays on and no keys are useable…i have tried different keyboards but with no avail.I have taken it to a computer repair shop and at first they said it was the harddrive and they replaced it..the machine worked fine for a few days and the same problem has come back…took it back to them and this timethey say it’s the motherboard…some how I am not convinced that the motherboard is the problem…pls can u help me with this problem

Thank you