Where is my core?

Have been practicing at home, but just doing "easy & gentle" poses.  So, after half years, I decided to go back to Yoga Limbs to get my strength back.  Last night, we were requested to close our eyes and do the practices for more than half an hour. This was different experiences.  Normally, we only pracitce tree, mountain with eyes closed. But once we close our eyes, the pose that we usually feel easy becomes harder and need more concentration. See, sometimes the things we feel certain is not that certain by its nature.

The hardest one is to do vinyasa from crane pose (Bakasana) to 3-points headstand, then to side crane pose (Parsva Bakasana). Havn't practiced so powerful poses for long time, the feeling is like "I should be able to do this, but cannot now".  The worse part is I hurt my nerves from neck to right shoulder.  I could not turn my body when I woke up this morning and had to see bone doctor on a No. 8 Typhoon day. Core became weak!  I need to get my core back.  I think the most effective way for me is headstand, 90 degree headstand, hold it as long as possilbe.