If the Smart tab 3G hangs on powered by Android, then it must sadly be that the system has sadly gone corrupt. You then have to try the following:
- Locate preferably the stock ROM (we will try and locate it for you as well). It can also also be Custome ROM but you will need to first flash in a custom recovery.
- Download and Install SP Flash Tool
- Download and instal the MTK vcom USB drivers
- Then flash it the ROM as per the tutorial here (only skip step 1 & 2.
Let us know if we can be of any further assistance.