if you reference the admin guide under "Manage Firmware", it does say that there are two type of firmware and in case of upgrade failed or crash, the secondary firmware (before the upgrade) will be loaded instead
http://www.cisco.com/c/dam/en/us/td/docs/wireless/access_point/csbap/wap5x1/administration/guide/WAP551_561_admin_guide.pdf

If you have the Backup configuration files for recovery, you can actually reset to factory and reload config - so far that is the best means as I am not aware any other means to go into it as it cannot even boot up
http://sbkb.cisco.com/CiscoSB/GetArticle.aspx?docid=a03797d066774f17a4ab80eedf1e5a2d_Backup_Download_Configuration_File_on_WAP551_and_WAP561_Acce.xml&pid=2&converted=0

I can't login to the UI... It has received an IP address, and I've tried to factory reset by pressing the rest button ... but still not working and can't still access the UI, only the log on screen

Thanks, I got it Working now...the long press on Reset did the magic, and by the way... It is very important to clear IE Cache, else it will not work.