This sounds like one of two problems. either you lost the vacuum supply from the engine to the HVAC system or the vacuum rotation valve has fallen apart.
Here is a guide and a diagram to show you what to look for:
https://www.2carpros.com/articles/air-vents-stay-in-the-defrost-position
Check out the diagrams (below).
Please let us know what happens.
Cheers, Ken
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Wednesday, April 25th, 2018 AT 10:19 AM