I was resetting my IMAC desk top computer, I must done something wrong and suddenly ? mark on my screen and I can not get rid of the screen? and I don't know where is hold botton in key board? please helpi

[?] indicates that no valid startup disk was found.

There is no 'hold' button. The instructions must be telling you to hold (verb) some other button, possibly the power button to force a shutdown.

You could try restarting while holding 'option' to check if the recovery partition is still available, but it sounds like something went very wrong with the reset and you need to reinstall MacOS.