All of a sudden, my digital signature can't be located and I get prompted to create a new one when I attempt to digitally sign a document even though my signature still exists (and can be seen) in preferences.
We apologize for the delay in response to your query.
If we understand your query correctly, you have created a digital signature but when you are trying to locate it you get a prompt to create a new ID. However, you already have it listed in the signature in Acrobat's preferences?
If this is the case, then you may follow the troubleshooting steps (referred to older version) suggested in the help article here- Can't find digital signature (Sign and Send PDFs) and check back.
Let us know if the solution provided above doesn't work for you.