Accidentally I short circuited 5v & gnd of R-pi, since then my R-pi is not working. How can I make it start again by means of replacing some IC or power regulator or anything ? Please help.... only act led is on...
I am trying to research "main polyfuse" tripping behaviour and so far I would be concerned about your RPi but not without hope.
If you have specifically shorted the 5 Volt connections (physical pins 2 and 4) on the 26 (or 40 on later Pis) to the Ground on Pin 6 this is a case that the polyfuse SHOULD protect against. At the voltages and currents involved, 5 Volts and say 2 to 3 Amps (or less) from a USB Wall-wart the device should recover (from the high-resistance value it goes to when it is heated by the excessive current flow under fault conditions) - most of the recovery happens within a minute or so according to Wikipedia but there remains a residue of "extra" resistance that can take much longer (hence the day that others will quote) to complete the recovery. Unfortunately that "extra" resistance is still enough to stop things working properly on the RPi - you will find other Q&A here about people experiencing issues that can be traced to lower than needed voltage levels from the Power Supply under various situations.
If you have a (Digital) Multi-meter you might - very carefully - find the
TP2 test points on the RPi's circuit board and measure the voltage across them when the Pi is powered up (even if it is not "working") - I can't recall the absolute figure but I think anything under 4.5 Volts on those two points (which is the +5V power rail AFTER the polyfuse [TP1] and Gnd [TP2]) is not good enough for the Pi to operate successfully.
If you can find the polyfuse (is labelled
F3 on my RPi) and have that DMM you may wish to try and measure the resistance on the meter's lowest resistance range with the power supply disconnected. The resistance should be almost un-noticeably different compared to the resistance value you get from shorting the two meter probes together.
I have a totally unproven theory that packing the RPi back in it's antistatic bag that it came with (to protect it from static damage) and then wrapping it up in a cool place in another bag to seal it from as much condensation laden air as possible, and popping it in a freezer might improve the recovery from the remaining previous thermal effects that were caused by the overload. However there is no experimental evidence, that I have found that confirms or disproves this.
One other point is that a short on the power supply may have corrupted some of the data on the SD card if it was being written to at the time or the RPi had file-systems open and had data that had not been flushed out to the SD card - which is pretty much all the time it is running...
tl;dr: Leave it alone, un-powered for a day (or wrap it safely and pop it in the freezer for a lesser time if you want to experiment) and try again after you have checked that the SD card is not suffering from data corruption.