What happens if the catholic church collects money for something specific, then does not spend that money on what they collected for?

You would need to talk with the people who takes care of the offerings in that church.

If they spend the money on something else

without some sort of notification and approval of the people who gave the money

it's called "fraud"

and if they are reported

they can be charged and/or sued.

So

for example

legally

they can report, "We didn't know there would be no snow this year (if that turns out to be true). The financial staff has identified X as the church-related thing that most urgently needs funding, and they have voted to use this money to fund that thing, which will mean that we do not have to have a special collection for that thing."

They could STILL probably be sued

but given that (in the example)

they have been completely up-front about the money not being used for the thing intended

and they have indicated that it saves the people who donated having to donate yet again

probably it will not be contested

though an individual donor could probably successfully sue for return of their donation, claiming money spent on something other than what they gave money for.

Catholics do it TOTALLY different from Methodists and Lutherans. And don't even get me started on Baptist snow-removal policies. And have you noticed the Bible doesn't say One Single Word about snow removal at churches? It is therefore an unscriptural man-made doctrine. And it involves works. That's bad.

The priests have a party with the money in the spring.

They'll use it for something else and then collect again.  They always collect again.

If you'd like to avoid it next year, offer to remove the snow for them and have them take up a collection only after the job is finished.  No more preemptive collections.

That all depends on the church. Regardless, you are not getting your money back.

That's what they have done forever. 

What they should do is put it to a vote. Save for next winter or use it for something else.

They are a law unto themselves

They use it for something else.