From the information you are giving it sounds like the tablet is stuck in a boot loop. It is getting stuck on the first screen and then powering down and restarting ans happening again, correct?
If that is the case, then the update corrupted some stuff! You can try doing a hard reset and if that doesnt work then it will need to be reflashed, which is a whole new topic.
How to factory reset. NOTE: You will loose ALL data!
Make sure the tablet is turned off (press and hold the Power button for about 10 seconds to turn it off).
Press and hold the “Volume Up” and “Power” buttons at the same time, when logo appears on the screen release the “Power” button and continue press the “Volume” button
When recovery screen appears use “Volume Up/Down” buttons to navigate and select “wipe data / Factory reset” by pressing the “Home” or “Power” button (Depends on Galaxy Tab model).