It’s easier to fix ’em than to buy new!
You will need cord that is close in diameter to the old cord so that it works smoothly with the hardware already on the blind. I have used string – it doesn’t last and tangles easily. I have used rope – its movement is restricted by grooves in the pulleys and the blind won’t raise or lower.
I like cotton clothesline which I sometimes dye to match. Polyester and nylon ropes will not take dye to the degree of color saturation I like, so this time I used cammo poly cord. Cut two  pieces. Use the formulas below to approximate the length of each. Notice the only difference in measurement is the addition of window width to the longer cord. This one should be tied to the side without the brake on the pulley. [Pulley brake is shown in pic 6.]
LONGER CORD=> double the height of the window, add its width, add 1/2 window height, and add 6 to 12 inches to allow for error.
SHORTER CORD => double the height of the window, add 1/2 window height, and add 6 to 12 inches to allow for error.
1) Your blind will be attached to the window frame or mounting brackets from the bottom of the mounting board. Usually store bought roller blinds have a valance hanging in front to hide the blind hardware. Unroll your blind all the way and look up under the valance. You will see mounting screws [like mine] or screws with wing nuts [easier to mount and remove]. Unscrew whichever you have to take the blind down from the window.
2) Roll your blind all the way up while tightening the draw cords. Ooops! …got my foot.
3) Blind draw cords are usually tied onto a ring or loop of some kind with a square knot, nothing fancy. Here they are tied onto a triangular shaped wire with a raggedy knot. Cut one side loose…BUT don’t pull anything yet.
5) Flip your blind over on its belly. You will be looking at the top – the plane that meets the top of the window frame when hung. [inside mount] Pull the cord through the blind material over the top of the blind and down the outside of the rolled up blind.
6) Continue pulling the new cord along the path of the old cord. When you reach the pulley attached to the mounting board, be careful the cord follows the grooved center and continue pulling until you have excess blind cord. The pulley with a brake is shown in this picture. You might have chosen the other side to start. That is OK. The procedure for both sides is just the same. The hardware’s appearance is the only difference.
7) Repeat this procedure with the other end of your new cord at the other old cord knot onto the other hardware piece. Start with Step 3. When you are finished, your new draw cord will be continuous
8) Now rehang your blind in the window frame and roll the blind material from the bottom up. You can raise and lower your blind with the continuous cord or cut it to the length you like and add beads or cord ends.