Ok, first a few questions...
when you changed roms, did you wipe cache and data?
Second, did you use anything like titanium backup to save your apps and settings?
third, can you access recovery?
If you have access to recovery, i would try to wipe cache dalvik and data and try a new rom and see what happens, if this doesnt work, here are some more options i would run through
If you can access bootloader, i would try to put the pc36img back on your sdcard and see if it will load.
Next thing I would do is to "revert to stock." These are a few great link that helped me when i had to swap my evo.
[TUTORIAL] How To Unlock Nand Protection ~ Part-2 [Gaining RW Access/Full Root] - xda-developers
Can NOT get RUU to restore my EVO to stock. - xda-developers
There is a lot of info here, so read and then try. Next time you root, use simple root after you downgrade. its is a lot easier. Let me know if you have any questions