Exit the power armor
Check the fusion core
Replace the fusion core if empty
Re-enter the power armor
Repair damaged armor pieces at a power armor station
Make sure all armor pieces are attached
Remove and reattach the fusion core
Fast travel away and back
Save the game and reload
Restart Fallout 4
Verify game files
Install the latest patch
Disable mods and test again
Load an earlier save
Use the console on PC to reset the armor if needed