By windows blinds we are referring to mini blinds, wood blinds or faux wood blinds, they all operate the same so that is what I will address. If your blinds we're ready made off the shelf from a big box store, trash them and buy new. Those types don't offer replacement parts. Yes, one could do a simple restring, provided you have the exact diameter polyester cord. Different manufacturers lift locking mechanisms only lock properly with certain size cord and they vary.
If the blinds we're custom ordered and are of a top manufacturer, like; Hunter - Douglas, Bali (custom)-Graber Springs Industries or Levolor then you should be able to call any dealer who handles your brand and they should be able to get parts for you. Those manufacturers all have a *Limited Lifetime Warranty (only to original purchaser and in original window), it boils down that the warranty is for the locking mechanism and the tilt mechanisms. The rest of the blind: the slats, headrail and bottomrail, if they get bent - its on you , the ladder and cords will deteriorate from sun and friction of being used, which is normal wear and tear and is not covered.
So if you have a quality brand blind, a flat blade screwdriver is the only tool you would ever need to repair a blind. It's all so basic anyone can do it. Two primary parts that I have seen go out are the tilt and locking mechanism. Mostly the tilt, especially wand tilts. One can take down their blinds every several years and put some drops of lubricant into a Wand tilt mechanism to help it last, or if having problems with a tilt mechanism that is getting hard to turn do the same thing. The newer old school Cord tilts don't have a problem.
Of all mini blind problems the main thing that does go out is the lift cord from raising and lowering the blind. Beside just everyday use, the other contributing factors for lift cord failure is that when raising or lowering the blind one needs to make sure the slats are in a completely open position. It helps eliminate cord friction and allows the blind to be raised up and down much easier. Also by keeping your hand with the cord in it up next to the blind when you are raising or lowering this eliminates cord friction. I have seen many stand away from the window several feet and open or lower the blind with the cord at angles that you can see rubbing at the metal headrail. This is what causes cord failure. It's funny that the ladder which holds the slats and is made of the same polyester material and is much smaller than the lift cord, and faces the sun more directly, never has a problem.
With a good quality mini blind and proper operation, a blind should last for 25 plus years easily. Many blinds I installed over 35 years ago are still up and operating perfectly.