Previously I upgrade my RB750 from 5.25 to 6.0 and everything works fine.
But this time my RB750 bricked after upgraded to 6.1.
Now when I turn on the power, it’s LEDs are always on, no flashing, no off, even I hold on reset button or conneted reset jumper hole.
Is there any way to save my RB750?
You must hold the reset button in and keep it depressed while plugging in the power. Keep holding for 30 sec. and then release. If this does not work, try using the NetInstall to reapply the firmware. If that does not work, you have a bad device.
Yes I had hold the reset button, but nothing changed. The LEDs are still always on.
What make me feel worse is that my RB750 works well for me over 1 year, now it got bricked just because one firmware upgrade operation.
MikroTik should do more testing before they release a new official firmware.
can u downgrade to 6.0?
No. My RB750 is totally dead. All of Ethernet ports have no response when connect to my laptop. The OS (Win7) shows the network adapter of my laptop is “not connected”.
do you found any solution?
I have the same problem! 
The solutions are - reset and netinstall.
Read carefaully how it is done :
http://www.mikrotik.com.my/reset-to-factory-default-settings/
It is very important when to press the reset the button and when to release it.
Here are netinstall instructions :
http://wiki.mikrotik.com/wiki/Manual:Netinstall#Configuring_RouterBOARD_without_COM_port
It is almost impossible to brick so much by upgrading that netinstall won’t be able to fix.
Please make sure you press the button before you plug in the power.
- press button and hold it
- plug in power and keep holding the button