If the LEDs are lit dimly, that usually means either power supply problems (ie. switching regulator, caps, ...) or CPU is dead/burned.
I would start with easiest thing first, replace all electrolytic caps - it's the most common problem with mikrotik HW.
Then measure voltages on all electrolytic caps to see if you have all required voltages (it helps if you have another working board you can compare with).
If any voltage rail is missing or different, switching regulator is dead and have to be replaced. For a quick test it's possible to run a board directly from lab powersupply, injecting all required voltages after switching regulators (ie on the cap pads), board should boot if it's OK and you will know it's just voltage regulator that needs replacing.
If all voltages are there and are correct, you can try to see if CPU is doing something - probe with a scope data lines going to flash to see if it's trying to access it.
If there is no activity, CPU is likely dead. You can try last resort hot air reflow, but it's rare these small chips would suffer this type of failure.
If CPU is trying to do something, it's possible flash is corrupted in some bad way... desoldering it and reading it may give a clue. It's possible to replace the chip, just make sure calibration block is copied from old one if possible (again helps to have multiple boards and chips so you know what you are looking at and what memory ranges change from device to device...)