Does the mining process starts all over again and I loose all the work my machine has done or does it start from where it get disconnected and do I get the rewards for that?
Yes, the mining will continue even if the internet goes off. But if you don't have internet your miner will not be able to communicate to the network and submit the Pooof of Work hence it will create a fork in the network. So nobody knows that you have mined but you, which is useless and will not earn you reward.
Mining makes no sense if you are in a small office/personal purpose computer. It will mine, but you will not get any rewards because the process is computationally difficult and there are powerful machines doing the same task.