MacBook Pro 2010 all of a sudden slow
There are quite a few causes for the SBOD (spinning beach ball of death), but you'll have to narrow them down to figure out what you need to do.

Possible Reasons:


You are maxing out your CPU
You are maxing out your RAM
Your hard drive is full
You have malware


Now, with what you put in the question, I think the most likely answer is that your hard drive is full. To check, you can do the following:

Go to "About This Mac"

  

Then click on the Storage tab:


If you have less than 2 GB free, it's likely that's the problem. If you have plenty of space, then the most likely cause is the CPU. But if it is the CPU, there is probably another underlying problem because it wouldn't start so abruptly. Try to think of anything you might have downloaded around the time it started to slow down.   

To figure out what's plaguing the CPU:


Launch Activity Monitor (/Applications/Utilities/Activity Monitor.app)  
Sort by "% CPU"


Then you can google around for the specific process screwing up.