On Orcad layout, on running autoroute board if it has finished, warning 'All Sweeps completed (or disabled)'. So running autoroute board again is disable.
If I unroute one some track, and running autoroute board again it is disable too...
I have also faced such problems in the past, what i think is u need to change some of the Autoroute settings, also if u delete one of the report files generated (whixh i think is a text file or another such thing) might help.