Well, go to Steam, right click on Arma 3 -> Properties -> Local Files -> and then check game files or something like this

Hello,
as Rexon said, try repeatedly verify integrity of game cache. If Steam reports that it detected 1 or more missing files and repairs them, and the message is show again, an antivirus may be the cause as they tend to delete the game executable as it's downloaded.

// I'll remove the first post to make this thread shorter as it contains same content as an attached file

it has worked, but now when I click on play it comes up with a new window with a red 'X' saying:

"cannot create DX11 device:

Adapter (NVIDIA GeForce 940M ) Fullscreen Resolution 1366x768, format R8G8B8A8_UNORM, refresh 60 Hz Error DXGI_ERROR_UNSUPPORTED

last night I was playing ARMA 3 and everything was fine but now this is happening

and after I exited out of this window, another popped up saying:

"Arma 3 has quit in unsual manner. Info:

Exit code: 0x00000001/1
Time running: ~ 00:05:14.4

https://community.bistudio.com/wiki/Arma_3_Unusual_process_exit

I'll reassign this to Adam, he will be able to help you with that error more that me.

thankyou

Hello, try updating your GPU and Integrated card (if you have one) drivers. Also install latest version of directx.

I have a new laptop, and the GPU on it is a NVIDIA GeForce 940M.
I haven't used anything else, and it doesn't make sense that its not working if I was playing ARMA 3 last night perfectly fine.

can I update these online or do I need to purchase from a shop?
I may not have directx, I will have a look now

my computer currently has directx 11

Yes, you can download drivers completely for free from manufacturers website.

what driver would you recommend?

and is the GPU that I am running suffice for playing ARMA3?

You can download NVIDIA drivers here: http://www.geforce.com/drivers

I would recommend getting Nvidia Geforce experience as it automatically checks for new drivers. (When you open the link its titled as Automatic Driver Updates)

UREAL GUYS!!!!
IT IS WORKING!

for some reason it must have disabled or disconnected my GPU, thank you so much for the support and the quick and efficient help!
you guys are awesome!!

is there any way I can give you a rating or feedback, because this was by far my best troubleshooting experience ever!!!
thankyou so much!