You need to contact Pella. My clients just got new Windows when that happened. The first ones like that couldn't be fixed. Never used them again. Hopefully yours are newer.
This post may help you with your blinds https://homeguides.sfgate.com/restring-broken-blinds-strings-52976.html
If your Pella blinds are between the glass panes and operated via a slide lever rather than the small twist dial, you've probably discovered that this was a frequent problem for other homeowners, as posted on blog sites.
Try contacting Pella Corporation headquarters for assistance, as suggested.
If your interior pane of glass permits access to the blinds, you may have other replacement options.