Get a fresh copy of Pixie Scheme, and of the Pixie Scheme world that you were using, from a backup disc (you did make backups, didn't you?), or from another source, and try again. The copies you have been using may have gotten damaged somehow.
If the problem happened when you were running other programs under MultiFinder, stop those other programs. Perhaps one of them does not get along with Pixie Scheme.
Carefully consider the possibility of a computer virus. There are no general rules to detect these: No two are alike. Magazine