Thanks for answers, guys.
As I just said - if I plug LAN cable in my laptop (MAC addres is same with router's MAC address, IP addresses assigns by DHCP), internet works fine.
ISP 100% has put some kind of filters or something like that.
I accent that ICS (internet connection sharing) is not working too. Before it was OK - my router was able te get out to WAN and ICS was working fine too.
I checked my configuration again, but there just can't be mistake, because I did not even touch it - internet just stopped to work.
Is the IP address they assign a reserved IP address like 10.10.x.x or 192.168.x.x? If so its possible they are doing NAT several times and when you NAT the final time it breaks, you can only have so many NATs occuring before it breaks.
It is also possible if they are assigning the same extenal subnet as your using on your internal. 192.168.0.x for example to you and your using 192.168.0.x for your internal it would also cause your same issue. If both the WAN and the LAN have 192.168.0.x for example then it couldnt route outbound, it would route to itself. A trace route would reveal such a loop, if you post your traceroute it would also help. From your PC and from the Mikrotik.
Another thing to check if they are giving you a real internet IP:
Log into the mikrotik, verify the DHCP client is receigin similiar IP address, subnet mask and gateway, if so try pinging from the mikrotik to the real world. Write down some real world pingable addresses from your PC before hooking the Mikrotik back online to try this.
Without seeing your config further help would be difficult without more information.