We can't help you without a lot more information.
What version of After Effects?
What language do you have your OS installed as?
What language do you have AE installed as?
What language keyboard are you using?
What do you expect the shortcut key to do?
What is it doing instead?
m reveals the mask property in the timeline.
mm reveals all mask properties in the timeline
f reveals the Mask Feather property in the timeline.
g selects the Pen tool which will allow you to create a mask on a selected layer or draw a new shape layer
Alt/Option and Ctrl/Cmnd will change the pen tool from selection to convert point depending on where the cursor is on the path
If this doesn't help and you had this working before we need details down to the last decimal point.
i don't able to perform mask path by pressing shortcut "M"