Can't access the startup manager from boot
Are you using a wireless keyboard? To boot from USB  you will need a wired keyboard. 

From Safe Boot Mode, or even from Recovery Mode, you can try to undo recent changes in the system, like the installation or removal of a kernel extension related to the graphics card. If that graphics card is one of the officially supported models, you can also  try to reinstall the operating system from Recovery Mode, without erasing the disk.