1. Restore to factory conditions. Small chance that's it's OK again.
2. If that doesn't help ask an estimate for repair.
3. Then choose between repair and buying another tablet.
I dropped my Samsung galaxy tab 3 on the pavement around October and now whenever I turn it on it 5 minutes later it completely shuts itself down (it is also now very laggy and will only shut down in a lag spike). What do I do?