If that's the case, the only way this diode could have affected the starter relay is if it failed in shorted mode. However, that's highly unlikely since diodes usually fail in open mode. Could it be that you put this diode in backward so the current is going through it instead of through the coil?

A simple test will be to remove the diode and see if the starter cranks. If not, the starter itself or the solenoid are more likely culprits.