I have a 2004 buick park avenue NA, and I am having problems with the AC. Recently, the AC will only come on if I bang on the dashboard. Can anyone tell me how to fix this?

What do you mean by AC? Compressor or Blower motor?

X2 ^- good question...
and I hope you are not banging on dash close to the auto climate control, where the LCD display is situated !

I am completely uneducated when it comes to AC, so I do not know which part of the AC. All I know is that when I turn on my car and then try to turn the AC on, it will not come on without me banging on the top of the dashboard.

And no, the auto climate control is way too low for me to be banging near that, and I have already replaced the LCD screen that went out long before I had the car. The new one works like a champ!

For the novice this is a very complicated system to diagnose.Even though it sounds like an electrical issue, I would want to know from an A/C specialist that the refrigerant in system is within specs.
There is a way to check/ view any of the HVAC trouble codes...Key to On (I assume) hold the Off button and Auto and Recirc and hold them all together for 1 second. The trouble codes will display for 2 seconds each and then return to normal.
Do you have any stored codes?

The A/C compressor has a clutch and it will normally engage & spin when Auto and 60 degrees is selected.

OK, I should have been more clearer.......your car is running....you turn your A/C on.....

Do you have air flow at all speeds?

If you have air flow, does the temp change when you go from lo temp to Hi temp?

I will check! thanks!

I do not have any air flow unless I bang on the dash. But, once the AC is on, it does work for all speeds and all temperatures no problem.

OK, you want to remove the hush panel under the dash on passenger side, and also the glove compartment.....take a small hammer and tap the housing of the blower motor to see if motor runs....if it does need blower motor......if it doesn't, disconnect harness and attach test light to see if it lights up.....

If it doesn't, grab the connector harness at the blower control processor, and pull on it and see if the test light lights...