How do I open a pdf file in Acrobat DC. The icon in the creative cloud list of apps appears frozen.

I am a brand new user. The first time I opened a pdf the file opened Acrobat DC and I was able to add a signature. Two days later I am trying to do the same thing but it only opens in preview.The total list of installed apps in creative cloud do not work when I click on them. What am I missing. All help videos only address things about after one has got into an app.