Hmm - OK - so the site can be seen and looks normal - you just can't do stuff in the backend .
If you goto frontpage manamger - you are stuck - have you tried going directly to eth article via content / items drop down (and are all items locked?) .
A possible solution then will be in :
1. checking permissions - usually 0755 for directories (folders) and 0644 for files on the web server
2. I'm assuming you performed the global checkin and have superadministrator access to the backend of joomla - otherwise this will need to be set in config.php?
3. if all files remain locked - you can directly go to teh database with a client like navicat - havea a look aroudn and probably unlock them there - but you'd really have to look and see what teh problem was first