Question : Problem: PC Fan Very Loud

A few minutes ago my PC fan became very loud.  What does this mean and how do I fix?

Answer : Problem: PC Fan Very Loud

Depends ...

First, I'm going to assume you're referring to the fan on the CPU heatsink.   There may be other fans that could be causing this noise ... most notably on the graphics card;  but from what you've described it's probably a ramp-up of the CPU heatsink's fan speed.    

So, the possibilities:

... something could have simply failed;  although that's relatively unlikely here;

... if the fan is still working okay, then you most likely have a thermal problem with the CPU.   Many BIOS's control the speed of the CPU heatsink's fan based on the temperature of the CPU ==> as it starts to get hot the fan speeds up ... from what you've described it's running at its maximum speed, so the CPU is probably very warm.

This speed-up may be because you suddently started running a very CPU-intensive operation and are generating much more heat than normal ... in which case it will slow back down when that program finishes;

... or it may be because the cooling efficiency has been lost due to excessive dust in the system.   I've seen many cases where the heatsink gets covered by a "blanket" of dust ==> which almost totally eliminates its ability to eliminate heat.   This would easily cause the fan to ramp-up to full speed as it tries to overcome this "heat blanket" effect.

I'd suggest you open your system's case and look at the CPU heatsink to see if there's an obvious amount of dust.  If so, do NOT attempt to remove it with your fingers or a vacuum cleaner blower ... too much potential to cause static damage.   The best way to clean that out is with a can of compressed air that's designed for that purpose ... available at any OfficeMax, OfficeDepot, or major computer store.