Yours sounds like an intermittent problem so my experience may not help you...
If it is an A19 globe from LiFX it is reset by turning it off and on 5times. It should then flash colours then stay on white.
I found that the LiFX app on the iPhone was not able to reliably connect the globe to my wifi. The only way was to go to the iPhone wifi settings and select “Connect new device” (or similar words - if this option is not available then the globe is not broadcasting its presence and is either faulty, out of range or already connected to a wifi). The globe should appear in the resulting list. Before proceeding, change the name of the globe from the default to a simple unique one such as “readlight” (I found this tip on an obscure website). Now the iPhone should find the globe and connect it to your wifi. Then start the LiFX app, scan the Homekit number code for the globe (back of the booklet that came with the lamp) and it should then appear on the Homekit home screen.
Have fun seeing what commands Siri is able to understand for controllong the light.