Hi salleee
Most likely a Windows Update has changed your file associations
Tight click a PDF file, choose 'Open With' - Choose Another App
Check the box marked 'Always use this app to open pdf . . .'
If Acrobat is not in the list, scroll to the bottom and click 'More Apps'
Again if Acrobat is not in the list, at the bottom, click 'Look for another app on this PC', then navigate to find the Acrobat in C:\Program Files\Adobe and select that