Download file suddenly stopped working

I've been selling downloadable products for 3 months now without any problems. Now for some reason when a customer downloads the file the zipped file downloads in a second and is empty. I checked on the server and the full 20.6 mg file is still on my server. I haven't changed anything else in the cart or on the server since I first started the store. I reloaded the original file, hoping that would help. But nope, the file is still downloading as a less than one mg file and
an empty zip. Any help you can offer would be great!

Thanks!

I also posted this in bug reports, but I don't have a solution so any help anyone can offer would be great. I just got another order!

MODERATOR NOTE: removed duplicate post from Bug Reports area

The key to your problem is the root cause behind the word "suddenly".

If you didn't change anything, and things suddenly stopped working correctly after functioning fine for a notable period of time, then something beyond your control has changed.

Places to check:
a) did your hosting company change anything on the server? permissions, operating system update, PHP version change, security system changes, proxy system, caching system, etc

b) is the customer who is having trouble using a less common browser than others for whom it has worked fine?

c) can you replicate the problem on demand? in which browsers?
If you clear your browser cache and cookies, does it change the behaviour at all?

It was a permissions problem. Now I need to go talk to Bluehost and figure out why they changed my permissions out of nowhere. Thanks for your help. I'm sorry if this was a simple question I just didn't really even know where to start when my customers are suddenly complaining that they can't download their files! Not too happy a situation for an internet business. At least now I'll know where to look if this ever happens again.
Thanks again!

UPDATE:
Apparently my host does not allow permissions to be set at 777 so they changed them to 0. I changed the permissions back and it's still not working. Is it possible Zen-cart isn't aware that I changed the permissions back?
Is there some sort of zencart refresh I can do?
I can definitely replicate the problem on demand and so can others on other browsers and computers.

Thanks for any more help anyone can offer! I have an order hanging on the line!

If you can't get them to allow you to use 777 for temporary download files, then you'll have to change your download method.

Set these, Admin->Configuration->Attribute Settings:
Download by Redirect = false
Download by Streaming = true

Thanks! So far so good. Let's hope nothing else goes screwy with the hosting etc.