when i connect my hdmi cable, i lose control of computer

Just a guess, but it sounds like at some point your external screen became your primary screen and reasserts itself whenever it is active. Whatever effect the mouse would generate would be on the primary (inoperative) screen unless you were lucky enough to find the edge segment common to the two screens.

I don't know that it works for your model MBP with Lion, but at one time command+F1 would toggle mirroring on and off if two screen were connected; at an earlier time, it was F7 on some models. That would get you back the menu bar and cursor, but not access to the arrangement tab in the displays preference panel to move the menu bar back to the internal screen.

This is Extended desktop. Many users find in a case like this that the mouse cursor and MenuBar are on the OTHER display.

The "Gather Windows" button is supposed to put both sets of controls on both windows.

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Move the mouse cursor so that it is on the visible screen. That will typically be to the left, but it is possible it is to the right.

Use "Gather Windows to get some controls ontot he screen. Maybe you can manipulate which screen is primary as well (by dragging the menuBar over).