Hi. After I update my OS, my MacBook is unable to open

Follow the Advise of previous posting.

If that should prove unsuccessful - restart in Safe Mode and again, while connected to charger. Safe Mode does various routines behind the scenes so loads slowly - normal.

This may or may not allow the upgrade process to finish and present the Setup Assist of Big Sur.

What do you mean by open?

Usually an update will go through a series of restarts before it is completed. If you shut it down manually while the restart is happening, it may be confused where its at. Just hold the power button down again, and make sure the computer is connected to power.