Which illustration are you trying to open ,
HAve you tried creating the new document.
Where is the fie saved , is it happening with one file ar all the file?
trying to create a new document- unable to
You may try the following (you may have tried/done some of them already) and see whether it helps:
1) Close down Illy and open again;
2) Restart the computer;
3) Close down Illy and press Ctrl+Alt+Shift/Cmd+Option+Shift during startup;
4) Move the folder with Illy closed;
5) Look through and try out the relevant among the Other options;
You have probably been through 1) and 2) already, and 5) is more unlikely.
Even more seriously, you may:
6) Uninstall, run the Cleaner Tool, and reinstall.