I use my work email address as my Apple ID, and have it set up more than a year ago. I have had this same email address for almost 15 years and it is still functioning. Today when I went to sign into iCloud it says incorrect ID or password. I first tried to reset my password, but it tells me the email address is not a valid Apple ID. I thought that was odd, and tried to set up the email address again as an Apple ID, but now it tells me that is not available.
There are several reasons why you can't setup a new AppleID with a previously used email such as it's used as an emergency contact / backup account for another AppleID and verified or it's already the primary account.
No matter what the case, your best bet is to follow precisely the steps at:
There is a link below if you can't unlock the AppleID you think you had titled "Forgot AppleID" at the end of that online tool, there will be a link to contact Apple Support - they are truly the ones best suited to help you with your specific details. They can look up purchases, talk with you to determine your AppleID and help unlock if needed.