Loss of fitness can happen for too many reasons to even start listing.
Any change in diet, exercise regiment, sleep, non-workout workload, can have adverse effects. I tend to have a very sharp loss of power after consistent days of inconsistent sleep. That is - 4 hours the night before - 8 hours the night before that... etc... When I have everything dialed in however, I get back to my peak fitness pretty quickly.
The other factor that has changed how my body has been reacting to these stimuli has been age. It's funny how you FEEL so different at 23 opposed to 29. It feels like it happened overnight. I consider myself on the cusp of youth and fading into middle age not because of a number necessarily - just things like power, recovery, and general overall health are not the same anymore.
TWENTY-NINE!!!!??? You could be in for a long fade fella!