PC
1. Make sure the necessary midi cables are correctly connected from the midi in and out of the external midi device (keyboard) to the midi in and out of the midi interface.
2. Reinstall the midi interface/sound card drivers.
3. Go into the Setup MME and make sure the necessary midi ins and outs are not set to inactive.

Mac
1. If not using OMS, make sure in the Midi System setup that the appropriate modem or printer port is set correctly.
2. If using OMS, make sure Cubase is unchecked under the "inputs" in the midi system setup. In the same window, make sure "enabled" is checked and that "Thru off" channel is set to off.
3. Consider rebuilding OMS studio setup.