Error: Could not read Block X of relation base/Y/Z
Before doing anything else, read and act on: http://wiki.postgresql.org/wiki/Corruption . 

Most likely you have disk or file system problems. 

If you suspect any kind of DB corruption for whatever reason you should stop the DB and copy the entire database at the file system level before attempting any recovery.

Once you've done that, then you can look into possible repairs. You'll probably have some significant data loss, so your goal should be to get it working to the point where you can pg_dump the damaged databases, re-initdb, and reload.

If you have a recent backup, now would be a good time to think about using it.