HI, I connected my pc to rb750g eth2 port. 3,4 dont work, I dont know why. Just 1,2,5, but 1 is supposed to be wan. Dhcp server of rb750g cant give my pc ip address. I tried to reset it, i hold reset button for 20 secs but nothing happens. Can you tell me what can be wrong? Is that normal? How to access it? What is default ip of rb750 on ports 2 or 5?

Sounds like the images / packages on there have gotten messed up. I would try a netinstall, then if you have a backup of your config restore it. The default IP should be 192.168.88.1 for a default config.

http://wiki.mikrotik.com/wiki/Manual:Netinstall