If cleaning your powerhead and using vinegar water does not help, you either can get it fixed or replace it. I am lucky. My LFS will take a look at anything mechanical such as powerheads, etc., and then, they tell me what is wrong and will repair it for the cost of the part. Now, they will be more expensive than, buying the part on the web, but I like the fact, that, I just have to pay retail for the part, and they install it and fix it for a great price...
Another thing, I do, is when a reefer is getting out of the hobby, I try to buy pumps, heaters, etc., at great prices, so I have extras, just waiting when something goes...