SCL and GND connection
What happens if SCL of I2C is connected to GND?

The bus won't work.  This should have been obvious to the point of this being a dumb question.

Data is transferred synchronously with the clock.  When the clock is shorted, there are no clock pulses.  It therefore follows that data can't be transferred.  Again, this should really have been obvious.

Shorting either IIC line to ground does no physical damage.  That is because both lines are passively pulled up and actively pulled down.  Devices only drive low, not high.  There will be no excessive current or any damage.