it you can load/save other projects, sound like the project is somehow corrupt.
there is a backups directory, which should have the last saved version.
you can copy that to the top directory (with new name) and see if that loads.
but your best bet is to contact squarp via their contact page…
you can then provide them with the project directory and they can take a look to see what is wrong, and if there is anything to salvage from it.
also they can hopefully tell you if its was caused by a bug, or a hardware issue (e.g. with the sdcard)
really given its just an error message there is nothing else to go on, except trying the backup.
Ive detailed in the past more information about project structures etc, if you want to dig into that, but it’ll unlikely yield anything useful
so… back to point 1 
… contact squarp via thier contact form