Today, I've tried to upgrade my boot loader, I have RouterOS 5.16 installed and under System -> Routerboard ,
I've seen possibility to upgrade to newer version, 2.38, I clicked upgrade and after that reboot my router as suggested by info windows. The router doesn't boot anymore, what could go wrong, how to solve situation now.

Thank you in advance.

Have you connected a serial cable and seen what the router has to say?

Use the reset button on the board while you power it up. This will load the backup bootloader. This is documented in the manual.

-- Nathan

I had Smart UPS connected on RS232 port and had in my configuration:

System -> Routerboard -> Settings "Enter Setup on any key"

I disconnected my Smart UPS and RB1100 booted as usual. Thanks to forum member ludvik, I was able to solve my problem.
http://forum.mikrotik.com/viewtopic.php?f=2&t=51101
It look like that Smart UPS signals were causing RB1100 to go into setup mode. In case you monitor UPS with RS232 connection, you should have in your settings:

System -> Routerboard -> Settings "Enter Setup on DEL key"