Netsys stops responding - Run time error 5

After apparently setting up a chilled water network successfully and sizing it I attempted to amend a pipework section of pipe. After deleting a section I attempted to redraw a pipe, after the first click a pipe appeared extending off the screen and the wheel of dome appeared, and eventually the project stopped responding. Attempting to size now produces a run time error. The problem appears to be on one floor of a three floor project. Can anyone help please.