TRiLOGI version 5 and version 6 files are not compatible. If you want to edit a .PC6 file in TL5, first use i-TRiLOGI version 6 to save the file with the extension .PC5. Then you can open the .PC5 file in TRiLOGI version 5.xx.
It does sound like you have a corrupted file.
There are several auto backup files in the same folder as the original .PC6 file called "backup001.pc6", "backup002.pc6" etc. Try to look for these files and open with TL6 and you should be able to recover most of the program.