If you originally purchased it as a download from adobe you may be able to redownload it by logging into your adobe store account. However, adobe only keeps software links there for a limited period and it may be too late, in which case (or if you had discs and lost them) you're out of luck. Always save the original media--don't just run it and dispose of it.
Where's the download link?
Go to the main page of adobe.com, click on my adobe and work your way through the links. However, I doubt you'll still find it. Links are usually only available for three years.