I tried to upgrade firmware on RB450G and it failed, almost bricked the device.
Commands was
/system routerboard upgrade
/system reboot
No error was issued during procedure, but, after reboot, NIC leds were off and both blue and green leds were solid on.
Previous firmware version was 2.40 and new one 3.02.
Since then the equipement can't be accesed via Windox, web, ssh or telnet. Not even via serial console and I tried with every posible speed between 9600 and 115200 bps - putty window remain black - no output.
As atachement image show I tried to restore the device using netinstall. Every netinstall attemp was succesfully. I tried multiple versions 5.20 to 5.24 and even 6.0rc11 but after reboot routerboard does not boot properly.
It seem I have to repair the bootloader but I don't know how.

What botter me is the device just returned from RMA a week ago - capacitors problem. Since RMA taken over two months I rather prefer to buy other mikrotik board if I decide to use mikrotik further. It's a great device for it's price but fiability rise many concerns, for me at least.

Is there something else I could try ? Am I doing something wrong ? Is upper firmware upgrade procedure obsolete/incorect ?

Try booting with backup bootloader.
Here is how from RB450G manual

There are two boot loaders present on the NOR flash memory chip. The main one, that is executed by
default, and the Backup, which is built-in failsafe. In case something goes wrong in the upgrade process, or
you have set some incorrect settings that render it unusable, you can load the Backup boot loader by
holding the Software Reset 1 button (S301) connecting the power, and then releasing the button/jumper.
The Backup boot loader uses Default settings, which can not be changed. It is also not possible to upgrade
it. In case the Main Bootloader is damaged, RouterOS v3.23 supports Permanently setting the Backup Boot
Loader as Main (thus avoiding the need to hold the button).

About serial console - you have to open serial console session before booting your RB.
About RMA - of course, if it's a hardware issue then you have to do RMA again... Actually - I think MikroTik could make their products a dollar more expensive and use all solid capacitors (IMHO cheap and buggy hardware is an effective way of loosing current and potential customers)!

I solved it! Thank you very much!

It seems that I should press reset button for at least 10 seconds to activate second boot loader. Less than that - no effect - that was the issue regarding no output on serial console. After that a simple boot loader reset and a restore of boot order to nand-if-fail-ethernet restore the device to working state.

However, as a precaution measure, I maked a hole in a top of the case and installed a 80mm vent. I switched power source with an 12V 1A one and powered the vent from internal header. Temperature droped with 27 C. Wow! Also intend to change the capacitors with solid ones but I have to find a good quality ones.
As you said: I happy pay 20-30€ more than I have to buy one device per year if it fail due to doubtful quality components.

Thank you again for your support!

Deleted because not related.

Ok. Done that.
Hope that Mikrotik people will take suggestions into consideration.

The problem that have caused me a lot of troubles, was upgrading from 2.41 to 3.xx routerboot firmware.

I experienced in 750s, 2011UAS-RM and 450s

reverted bootloader to 2.41 and latest ROS and they are running fine.

The best "practical" method which always works for me is:

a. Start Netinstall with Mikrotik deviced OFF (not powered on).
b. PRESS the reset button and KEEP IT PRESSED WHILE powering the Mikrotik device UNTIL you "see" it in Netinstall program.
c. THEN RELESEASE the reset button and you're ready to flash it.