Brake oil came out of the piston of my Tektro hdc 300 and the brake is not functioning. What to do? One day, I tried to disassemble my hydraulic braking system (Tektro HDC 300) having a goal of dipping the brake pads in alcohol. Well, I had somehow managed to separate the pads and the spring like metal object. But at one stage, after I squeezed the brake lever for a couple of times, one of the two pistons came out and all the oil got spilled... now my brake doesn't work at all. How can I fix it?

It can be noted that I feel no resistance while pressing brake lever? Is this normal under this circumstance? Will I have to change the whole brake set, or will just refilling with oil be enough?
If you are going to do it yourself - you will need a hydraulic brake bleed kit.
You basically need to replenish the fluid in the hydraulic system.
This will either be mineral oil or Dot brake fluid.
Check with your brake manufacturer which it is.

There is a technique and procedure to bleeding brakes as well. Weigh up the cost of doing it yourself - or just paying an experienced cycle mechanic to do it. Its a fairly standard job and shouldn't take long at all - especially if it is only one brake.
Most hydraulic brakes are maintenance free - in that the system is sealed and air does not get into the system.

Unfortunately, you found to your cost - by removing the pads and pressing the levers - you popped the pistons out of their seating (and seals) which leaked all the fluid from the system. A tip would have been to wedge something between the pistons of the padless caliper - in case you accidentally knocked or pulled the brake lever.