I guess everything is in the title. I wasn't trying to connect any pins. It sounds stupid but I was holding my keys and was just trying to put the usb power supply on the Pi and accidentally connected 3.3v and 5v or both 5v (don't really know). Now only the red LED is on. Is my Pi dead?
As mentioned by others it seems like you may have shorted your device. I would suggest following some troubleshooting such as here to potentially resolve your issue if that is not the case. There is a section titled "Red power LED is on, green LED does not flash, nothing on display".
However you most likely need a new device or need it to be repaired if you shorted your device.
The expression of "letting the smoke out" is just that, silly expression , and is based on believe that each device is filled with smoke during manufacturing.
An unprotected device, such as protection against incorrect DC polarity with "an idiot diode ", will fail in nanosecond.
"Troubleshooting" hardware such as yours after the mishap is waste of your time and resources.
Get a new one and learn from this experience is the best you can do.