Let me first put forth in brief the config of my hexacopter.
It's on a 550 frame with 2212 1000kv motors having 10*45 props.The flight controller is Pixhawk 2.4.8 with Simonk 30A esc powered by a 6200 40c lipo. I have assembled the system and "ALL IS WELL" untill the copter is armed.The main problem starts when the throttle is raised , the copter tends to sway one side and topple. Further increasing the throttle gives a take off but copter doesn't respond to the controls and hence "safe landing " is a dream , it was a crash both the times I tried.
Assistance expected and thanks in anticipation.

Hi ghemant!
Welcome to the forum!
I hope the damages are so far less! Plenty more to come so be prepared! (Note : it's better to fail now rather to fail on an important day.)

Jokes aside!

Safety note: while working with multirotors or any aircraft Please remove your props when you're not flying and doing something on the airframe!

Check for the following!
1) recheck if you've selected the correct hexacopter frame type
2) recheck your ESC to pixhawk connections

The above two if done wrong will result in sway in one direction and topple.

3) redo your ESC calibrations on the pixhawk. ( Remove props and do)

4) check that the radio is properly calibrated on the mission planner and also if you give any stick inputs then the slider should also move in that direction on the radio calibration page.

The above two points should take care of no control issue!

One last thing is always before updating the pixhawk firmware,use only stable firmwares!

Do check this link
https://ardupilot.org/copter/
If you follow it you should have a well working hexacopter!

Personal advice/experience: Don't take any short cuts or rush things with multirotors and pixhawk, things get ugly really fast!

Thanks Naved Sir,
I have tried doing the measures quoted by you but will redo the same in the sequence mentioned.
Get back to you soon.
Thanks

Looking forward to it!

Sounds like a radio callibration issue. Are you sure that at mid stick, the radio callibration bars on mission planner reads 1500 ms. If not then adjust the trim to make it 1500 for A, E, R channels.