Just started to use 2017 sp4.1 and its corrupted two files already, that have been fine since 2004!
Said the files were corrupted and I could try to save as a different file, with I tried and then this file encountered a serious problem when I tried to open it.
Restarted Solidworks and now things seem fine again........
Could be a one off, but I have never seen this before!
The old solidworks session was less than 3 hours long.