There are many places in which systematic reviews are published. The majority probably appear in the journal literature, but there are also organizations that publish SRs as reports without them every being indexed by a major literature database such as PubMed/Medline.
Also, it doesn't mean you cannot complete the review you wanted just because you discover a prior review. There are several good reasons to go ahead with yours. If you find a prior review, evaluate it!
You may find after reviewing the prior review, that your idea is still viable. Also, you can use the prior review in several ways:
If there is a similar review, be sure you mention it in your introduction. It can be part of your rationale:
A scoping search was performed to determine if a prior systematic review had been done on X, Y, and Z. Whosit and Whatsit published a systematic review in 2002 that found (major findings here). However, their study precede the development of the new and improved intervention that my systematic review will include.