Error: You have no Oracle client installed

i am using oracle instant client 10.2.0. and toad 9.0.0.160. till yesterday every thing was working fine . but since today morning, suddenly Toad has started to throw the error “You have no Oracle client installed”. every system parameter is same as previous including path, oracle home and tns_admin. i have also tried to reinstall the toad but this did not help . please let me know solution for this.