I dropped my acer aspire 5742z and now the fan is noisy and it keeps turning itself off

Fan is noisy because it is probably broken. Laptop turns off, because fan isn't working at the full capability that it should.

When a Processor overheats it turns off. (BIOS turns it off)
This is a fail safe feature built in to keep the Processor from burning up.
(Sometimes Literally!)

{The Processor is the 'Brain' of a computer. No Processor operating, no computer }

Suggest you start by replacing the fan.
(Also while you're in there clean the cooling system, and the rest of the inside of the laptop. Use Q-tips, a soft small brush, {Makeup brush?}, and a can, or two of compressed air for computers )

The cooling fan is incorporated inside a Fan Assembly.
Fan and outer shroud. (Cage)

http://www.cdsparts.com/p-3156-new-acer-aspire-5333-5733-5733z-5742-5742g-5742z-5742zg-cpu-fan.aspx

The Fan Assembly is mounted to the Bottom of the motherboard.
This means the entire laptop has to be disassembled, and motherboard removed.

Do I think you can do it? Yes.

1) Remove ALL power. Remove the AC adapter (Charger) and Battery.

2) I suggest laying a clean, static free towel on a table, and work on that. (Prevents scratching the laptop, and the table)

3) FOLLOW Anti-Static Precautions.
Your body carries Static electricity. Static WILL fry out (Short Circuit) the delicate hardware components inside a computer.

Buy, and wear an ESD wrist strap. (Electro Static Discharge)
Average cost is around $3 to $6. This is one example of an ESD wrist strap,

http://www.radioshack.com/product/index.jsp?productId=2103245

Connect it's alligator clip to a good ground source.

(I connect to an unpainted surface, of the metal frame of an open, empty desktop computer case.
Not everyone has one of these babies laying around.

If the table you are working on has metal legs, connect to an unpainted surface of one of them.
Or use a large metallic object, and place it on the table.
Perhaps a large metal serving tray, or large metal knickknack. (Unpainted ]

4) Use a multi-compartment container for the various screws you will remove. Label each compartment for the area the screw/s come out of.

If no multi-compartment container is available, substitute with a clean egg carton. Label each egg holder. (Or even a piece of double-sided sticky tape. Make an outline, and label each area)

SOME screws look VERY similar to other screws.
DO NOT mix them up.

5) Make a record as you disassemble. Make notes, and drawings, (Legible) Perhaps also use a digital camera, or video camera. Use good lighting.

It's nice to have a Service Manual, but even nicer to have a personal record you created, along with it. Something to refer back to in case you are interrupted.

Acer Aspire 5542Z Service Manual link, (Free),

http://www.tim.id.au/blog/tims-laptop-service-manuals/#toc-acer

Drop down to the 'paragraph' that starts with Aspire 1200. (1200 in blue)
Go down to the 10th line in the paragraph. (5739 in blue)
Scroll across, and click on 5742G.

This model is VERY close to the 5742Z.

Also view this pictorial disassembly guide for an Aspire 5521/5551G/5551,

http://www.insidemylaptop.com/disassemble-acer-aspire-5251-5551g-5551-series-laptop/

Gives you a hands on look at what you are about to perform.
I would also like you to scroll down to the Steps 8 through 13.
This shows more in-depth info, on how to disconnect the Keyboard Cable.

Pay special attention to the photos in Steps 11 and 12.
The Locking Tab MUST not be completely removed!
Doing so will break it, and may be a cause for motherboard replacement.

The Locking Tab only moves about 2mm, or a little more than 1/16th of an inch. (That is about the distance of this letter L, is tall )

WHEN you get to the Thermal Module;
(Thermal Module is the Fan Assembly, copper curved Cooling Tube, and finned Heatsink attached to the Cooling Tube, (and on one side of the Fan Assembly)

I Do Not believe the entire Thermal Module has to be removed, in order to replace the Fan Assembly.
The Fan Assembly is mounted to the motherboard with two screws, and should remove just by itself. (Without removing the Cooling Tube/Heatsink)

Here for any additional questions you may have. Post in a Comment.

Regards,
joecoolvette