Operating system update to I thought “Mongoose” on MacBook failing.

I have received an error when updating my OS that reads “an error occurred preparing software update” . When I select “OK” to acknowledge the I'm redirected to “Choose Startup Disk” and directed to “select the system you want to use to start up your computer” with a constantly spinning wheel in upper right, and no choices ever displayed. Below this is a choice to restart this computer in “Target Disk Mode”. I don’t think that’s a good choice, can’t move past this, and can’t complete update, go back or use my MacBook. Please help!!!