Cord in roof blind snapped

Google and You tube is your friend...ive repaired two Dometic/Seitz ones in the past but that was for the little plastic clips breaking rather than the cord itself.

they are repairable...very expensive to replace....

if the cord is broken you will need some new cord as it has to be the right length to fit in the tensioner...just knotting the cord will probably make it too short...

try a few you tube videos and see how you go.

As BB says, can be done but fiddly and easy to break something. Unless you can see how it’s supposed to be corded, getting the tension right requires patience. You’ll need to identify the specific model - Leisure Spares   are good for parts and advice. Skyview Link

Also, you might find this LINK helpful. Seems some folks use waxed sailing cord as a replacement which makes a lot of sense in the absence of the original parts.