My Raspberry Pi is connected to the supply, and after shutting down, it is still connected to the supply and the red led is on. Is it possible to reboot the pi without removing the supply and without adding a button ?
The Raspberrypi doesn't have a power button, if thats what you're asking. You could Use a powerstrip with a switch. But make sure to always shut down your raspberry completely bevore toggling the switch, to preserve your SD card.
Or if you would consider adding a button nontheless, maybe you wan't to buy something like the pimoroni onoff shim ? I have no experience with it, so make sure you read the description closely to make sure it does suit your needs. Or google for alternatives.
Or if you don't want to solder, maybe this
You can install a momentary push button to one of the GPIO pings then add the line dtoverlay=gpio-shutdown to /boot/config.txt on your Pi.
It will shutdown the Pi but it will consume a small amount of power, about 35 mA, when in this state.
You can also press the button to power up the Pi afterwards. The default is pin 5 and ground but it can be changed.