Please help - unable to boot into mac recovery or usb after installing ubuntu 17

I installed ubuntu 17 on fusion drive iMac that has been dual boot OS X Sierra and windows 7. The wireless keyboard is not responsive at all. resetting SMC or PRAP does not work. Please, help me I need to be able to log back into mac or to be able to reboot from USB to format the computer and make clean install. After rebooting, it goes directly to ubuntu. I am not able to see anything at start up.


Thanks in advance,
Mohamed