2000 323i Will Not Reverse!

I have a 2000 323i with an automatic transmission. It's near mint condition with less than 88,000 miles, but today I put it in reverse and it wouldn't move. The engine revs as if it's in neutral, but it works fine in drive. What's my problem?

It's times like these "mint condition" becomes a very subjective term.
It could be your or fluid low in tranny, shift linkage out of adjustment, or bands inside the trans itself (in order of severity).
Here's hoping it is something even simpler.
Good luck!!